Feedback negativity (FN) is elicited when the outcome is worse than expected. Here we report that changes in outcome probabilities affect the reward prediction level reflected in the FN. We used a slot machine-like gambling task in which a button press produced one of the six outcomes: −10, −5, −1, +1, +5, and +10. The outcomes were first presented with equal probability, and subsequently, either +10 or −10 was presented more frequently. The smaller gains elicited an FN when the largest gain occurred frequently, whereas the smaller losses continued eliciting an FN even when the largest loss occurred frequently. The reward prediction level appears to be easy to raise but difficult to diminish. This disposition helps overcome a suboptimal situation.