It is known that oxytocin (OT) plays a major role in the regulation of social behavior including social recognition. We have previously reported that disruption of OT gene (OTKO) in female mice resulted in a failure of discrimination of familiar and unfamiliar social stimuli (Choleris et al, PNAS, 2003; GBB, 2006). Recently, we have found a similar behavioral phenotype in OT receptor knockout male mice (Sagoshi et al., JSAP, 2013). In the present study, we examined whether OTKO male mice might also show a deficiency in social recognition, using a newly developed automatic analysis system of social interaction which allows us to analyze not only social investigation (SI) but also other behaviors (SOSI Type 2; see Nagata et al., JNS 2014). Male OTKO mice and their wild-type (WT) littermates were transferred to testing cage to establish their home territory 48 h before testing. They were tested for social discrimination task that consists of two trials with 30min inter-trial interval. In each trial, an empty session in which two empty fan-shape plastic tubes were placed at two diagonal corners for 5min was followed by a 15min test session. In the 1st trial, an unfamiliar adult stimulus male mouse was introduced into each tube whereas in the 2nd trial, one of them was replaced to a novel mouse. During two empty sessions, both OTKO and WT mice didn't show any preference between two tubes and there was no genotype difference in total investigation time. In the test session of the 1st trial, although both OTKO and WT mice didn't show any preference between two stimulus mice, total SI time was significantly shorter in OTKO mice compared to WT mice. In the 2nd test session, WT mice sniffed the novel stimulus mouse much longer than familiar stimulus mouse during the first minute, whereas OTKO mice equally sniffed the two stimulus mice. Similarly to the 1st trial, total SI duration of OTKO mice was about a half of that in WT mice. Finally, total moving distance during test sessions was not different between genotypes. These results suggest a possibility that disruption of OT gene might affect social recognition partly by attenuating social interest toward same-sex conspecifics in male mice.