The Mid Niigata earthquake of October 23, 2004, is a shallow reverse‐faulting event with Mw = 6.6. We analyze transverse component accelerograms of the main shock and four aftershocks (Mw = 4.0 ∼ 5.5) recorded at 29 strong motion observation stations. The epicenters of the four aftershocks are located within 5 km from that of the main shock. The stations are well distributed around the epicenters and their epicentral distances are between 15 and 60 km. We calculate spectral ratio between records of the main shock and each of the aftershocks in order to obtain source spectral ratio by empirically removing the effects of wave propagation path and surface layers at the stations. Thus obtained source spectral ratio is inconsistent with that expected from the self‐similar omega‐square model. Fitting omega‐square source spectral ratio with variable corner frequency to the observed spectral ratio, radiated energy and corner frequency are estimated. Radiated energy from the main shock is 3.2 × 1014 J. Although energy‐to‐moment ratios for the main shock and the aftershocks are all distributed within a narrow range from 8 × 10−6 to 4 × 10−5, they definitely increase with the event size. This scale dependence is quite similar to that obtained previously for strike‐slip events in Japan. The relationship between the seismic moment, M0, and the corner frequency, f0, is M0 ∝ f0−3.3±0.2. These results suggest a break in self‐similarity.