False heartbeat feedback is a method to provide a modified heartbeat to the participant. It is known to be effective for relieving stress even without instructions. In this study, we compared the effects of false heartbeat feedback on stress reduction among three different modalities, audio, visual, and audio-visual. For this purpose, we developed a feedback system using mixed reality (MR) head-mounted display (HMD) and a wrist band heart rate sensor. Using this system, we performed the experiment where the participants were asked to move small beans from one dish to the other by chopsticks as a stress task. At the same time, they were provided with a false heartbeat in a different modality. The results of the questionnaire showed a stress-reducing effect in audio feedback. In contrast, the results of the heart rate variability (HRV) analysis suggested a stress-reducing effect in visual feedback. In both analyses, audio-visual feedback did not show a stress-reducing effect, suggesting that the combination of multiple modalities has a negative effect on stress reduction.