The Onikobe area is an active geothermal area situated in the Ou backbone range near the border between Akita and Miyagi Prefectures, northeastern Japan. There are Tertiary to Quaternary calderas and active volcanoes in this area. We calculated Q of coda waves (Qc) in this area by applying a single scattering model. Seismograms recorded at stations located within 10km of the epicentral distance for earthquakes which occurred at depths shallower than 10km were used in this analysis. The time window for the calculation of Qc ranges from twice the S-wave travel time to the arrival time of the wave reflected at the Moho discontinuity. We found a systematic variation of Qc with station location: Qc values were lower at stations in and around calderas than at other stations. The amplitude of the coda wave characterized by low Qc value decreased smoothly with the lapse time, while the decay rate of the coda wave with the high Qc value changed at around 6 s after the origin time. By taking these observational facts into account, we proposed a model in which the scattering coefficient was very large in the lower crust and a small and strong attenuation zone was embedded in the upper crust beneath a caldera. The synthetic coda envelopes explained the difference in the gross feature of observed Qc and decay curves.