Microwave pretreatment was applied to highly concentrated dewatered sludge produced from an oxidation ditch process (OD sludge) to improve its degradation and biogas production in mesophilic anaerobic digestion. In a batch experiment using methane fermentation, the biogas production potential of OD sludge was increased after microwave irradiation. The increase in the biogas potential depended on the irradiation period and the temperature. Continuous digestion of the pretreated OD sludge was carried out in mesophilic conditions using a laboratory-scale digester at 25 days of hydraulic retention time (HRT). Although the biogas yield in the control digester was 0.12 NL/g-total solids (TS), that fed with OD sludge pretreated with max 100 W of irradiation at 80°C for 60 min was 0.15 NL/g-TS. Subsequently, dewatered OD sludge was pretreated by a novel continuous microwave irradiation device at 400 W for 20 min and fed into a pilot-scale co-digester with six kinds of waste biomass. The biogas yield of the reactor was increased from 0.25 NL/g-TS to 0.28 NL/g-TS. Assuming that the biogas yield of each co-digesting biomass was constant, the biogas yield of the OD sludge was estimated to increase by 42%.