The mismatch negativity (MMN) event-related potential (ERP) is elicited by irregular sounds in a regular sound sequence. MMN depends on the length of the preceding regular stimulation: in repetitive sequences, changes following only two or three repetitions may elicit MMN. Thus, MMN builds up rapidly. Infrequent omissions in a repetitive sound sequence may also elicit an MMN-like waveform. In this study, the build-up of this omission-related ERP was investigated. Trains of two to nine tones with omissions at the penultimate stimulus positions were presented. Results showed no consistent omission-related ERP. In contrast, when omissions occurred with 9% probability among continuously presented tones, the omission-related waveform was elicited. Thus, the omission-related ERP builds up later than MMN.