This study aimed to develop a method to evaluate the sense of discrepancy to the operation response quantitatively. We examined the availability of event-related potential (P300), which is considered to reflect attention to stimulation, to evaluate the sense of discrepancy to the product response to the user's action. In the experiment using subjective evaluation and P300 to investigate the sense of discrepancy due to the lack of operation response (sound and vibration) to the shutter operation of the mirrorless single-lens camera, it was confirmed that P300 amplitude corresponds to the degree of the subjective sense of discrepancy. Our results showed that the P300 amplitude could evaluate the sense of discrepancy to the operation response.