We have revealed that there is a causal relationship between activation of dorsal raphe (DR) serotonergic neurons and patience when waiting for future rewards. To explain these behavioral data, we proposed a Bayesian decision model of waiting in which serotonin signals the prior probability of reward delivery. However, there is no direct evidence serotonin neural activity is modulated by reward probability. Five male serotonin neuron-specific GCaMP6-expressing mice were trained to perform a sequential tone-food waiting task that required them to wait for a delayed tone (tone delay: 0.3 s, tone duration: 0.5 s) at tone site and then to wait for a delayed food (reward delay: 3 s) at reward site. An optical fiber (400 mm diameter) was implanted into the DRN and serotonergic neural activity was recorded by fiber photometry. We prepared four tones (8 kHz, 2.1 kHz, white noise, and 4.1 kHz) and associated with four reward probability (100, 75, 50, and 25%), respectively. We focused on serotonergic neural activity during waiting for delayed reward. dF/F was the highest in the 100% test 23.4 ± 2.2% and gradually decreased 19.1 ± 1.9%, 15.9 ± 1.7%, and 13.5 ± 1.8% in the 75%, 50%, and 25% tests, respectively. In the 25% test, even if reward amount was changed, dF/F did not change (14.2 ± 1.8 and 13.1 ± 1.5% in one-pellet and three-pellet test, respectively). These results show dorsal raphe serotonin neurons encode probability not value of future rewards and serotonin response would be used to support flexible behavior.