Empathy is a multi-dimensional concept with emotional and cognitive components. The Interpersonal Reactivity Index (IRI) is a multi-dimensional scale of empathic traits. Although some researchers have attempted to translate the IRI into Japanese, these translated scales had limitations with content and construct validity, and measurement invariance. We therefore attempt to overcome these limitations by developing a new Japanese version of the IRI (IRI-J). We used three approaches to assess the validity and measurement invariance of the IRI-J. In Study 1, content validity was tested using back-translation, and construct validity was confirmed through a comprehensive investigation of a web-based survey using six other scales. Results indicate that the factor structure of the IRI-J was equivalent to that of the original version, and that the IRI-J had adequate reliability and construct validity. In Study 2, measurement invariance by gender was confirmed using data from four web-based surveys. These results suggest that the factor model of IRI-J for each gender is equivalent. The present study thus provides an improved measure of empathic traits for the Japanese population.