diff --git a/error.go b/error.go
index 4a92a567d..cc0aa9ecb 100644
--- a/error.go
+++ b/error.go
@@ -8,6 +8,7 @@ import (
 	"io"
 	"sort"
 	"strings"
+	"sync"
 	"text/template"
 
 	"github.com/fatih/color"
@@ -251,8 +252,10 @@ func (by byRuleNameField) Swap(i, j int) {
 // ErrorFormatter is a formatter to format a slice of ErrorTemplateFields. It is used for
 // formatting error messages with -format option.
 type ErrorFormatter struct {
-	temp  *template.Template
-	rules map[string]*ruleTemplateFields
+	// Note: Evaluating a template is thread-safe.  // > // https://pkg.go.dev/text/template
+	temp    *template.Template
+	rules   map[string]*ruleTemplateFields
+	rulesMu sync.Mutex
 }
 
 // NewErrorFormatter creates new ErrorFormatter instance. Given format must contain at least one
@@ -294,7 +297,7 @@ func NewErrorFormatter(format string) (*ErrorFormatter, error) {
 		return nil, fmt.Errorf("template %q to format error messages could not be parsed: %w", format, err)
 	}
 
-	return &ErrorFormatter{t, r}, nil
+	return &ErrorFormatter{t, r, sync.Mutex{}}, nil
 }
 
 // Print formats the slice of template fields and prints it with given writer.
@@ -315,8 +318,13 @@ func (f *ErrorFormatter) PrintErrors(out io.Writer, errs []*Error, src []byte) e
 }
 
 // RegisterRule registers the rule. Registered rules are used to get description and index of error
-// kinds when you use `kindDescription` or `kindIndex` functions in an error format template.
+// kinds when you use `kindDescription` or `kindIndex` functions in an error format template. This
+// method can be called multiple times safely in parallel.
 func (f *ErrorFormatter) RegisterRule(r Rule) {
+	// Synchronize access to f.rules (#370)
+	f.rulesMu.Lock()
+	defer f.rulesMu.Unlock()
+
 	n := r.Name()
 	if _, ok := f.rules[n]; !ok {
 		f.rules[n] = &ruleTemplateFields{n, r.Description()}
diff --git a/error_test.go b/error_test.go
index 3e5edbbcd..a860c8fc3 100644
--- a/error_test.go
+++ b/error_test.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"math"
 	"sort"
@@ -362,7 +363,7 @@ func TestErrorGetTemplateFieldsOK(t *testing.T) {
 	}
 }
 
-// Regression for #128
+// Regression test for #128
 func TestErrorGetTemplateFieldsColumnIsOutOfBounds(t *testing.T) {
 	err := errorAt(&Pos{1, 9999}, "kind", "this is message")
 	err.Filepath = "filename.yaml"
@@ -372,6 +373,20 @@ func TestErrorGetTemplateFieldsColumnIsOutOfBounds(t *testing.T) {
 	}
 }
 
+func TestErrorErrorToString(t *testing.T) {
+	err := &Error{
+		Message: "this is message",
+		Line:    1,
+		Column:  2,
+		Kind:    "test",
+	}
+	want := err.Error()
+	have := err.String()
+	if want != have {
+		t.Fatalf("wanted %q but have %q", want, have)
+	}
+}
+
 var testErrorTemplateFields = []*ErrorTemplateFields{
 	{
 		Message:   "message 1",
@@ -634,16 +649,40 @@ func TestErrorFormatterPrintGetVersion(t *testing.T) {
 	}
 }
 
-func TestErrorString(t *testing.T) {
-	err := &Error{
-		Message: "this is message",
-		Line:    1,
-		Column:  2,
-		Kind:    "test",
+// Regression test for #370
+func TestErrorFormatterRegisterRuleInParallel(t *testing.T) {
+	f, err := NewErrorFormatter("{{json .}}")
+	if err != nil {
+		t.Fatal(err)
 	}
-	want := err.Error()
-	have := err.String()
-	if want != have {
-		t.Fatalf("wanted %q but have %q", want, have)
+
+	rules := []Rule{}
+	for i := 0; i < 100; i++ {
+		rules = append(rules,
+			&RuleBase{
+				name: fmt.Sprintf("rule%d", i),
+				desc: fmt.Sprintf("description for rule%d", i),
+			},
+		)
+	}
+
+	done := make(chan struct{})
+
+	for i := 0; i < 100; i++ {
+		go func() {
+			for _, r := range rules {
+				f.RegisterRule(r)
+			}
+			done <- struct{}{}
+		}()
+	}
+
+	for i := 0; i < 100; i++ {
+		<-done
+	}
+
+	// Note: `syntax-check` rule is registered by NewErrorFormatter
+	if len(f.rules) != 101 {
+		t.Fatalf("not all rules were registered. %d rules were registered", len(f.rules))
 	}
 }