Many studies indicated that factors such as attention and motion play a critical role in time perception. However, it is not clear how subjective time for an unexpected event will be changed, compared with that for an expected event. The present study investigated this question by using two kinds of stimuli, one of them is the low-frequency oddball as the unexpected event and the other is the high-frequency standard as the expected event. In all trials, the standard was a square in line drawing and the duration was fixed to 1000 ms, whereas the oddball was a circle and the duration was set to one of seven durations from 500ms to 1100ms. After the standard was presented successively 4 times to 8 times (6 times on average), the oddball was presented once. Therefore, one session consisted of 34 oddballs intermixed with the 204 standards. Participants were required to estimate the duration for each oddball by using numeric keypad based on the magnitude estimation method. The results showed that durations for oddballs were estimated longer than those for standards. These suggest that an unexpected event causes subjective expansion of time.