This paper describes an objective evaluation method for the engine sound quality in a car interior during acceleration. Two principal factors, pleasantness and raciness, of the engine sound quality were found with a subjective evaluation test in a laboratory. Psycho-acoustic indexes corresponding to these factors were revealed by investigating the correlation among subjective ratings and acoustic characteristics. The index of raciness was originally proposed for the assessment of sound that makes driving fun when the sound is emphasized. We propose that the design of engine sound is required with consideration of the balance between pleasantness and raciness.