Polyrhachis lamellidens is a temporary socially parasitic ant whose new queen utilizes other ant species in the early stages of colony foundation. Field observations and rearing experiments suggest that Camponotus japonicus is a host species of P. lamellidens. It is presumed that Camponotus obscuripes is also a host of P. lamellidens by rearing experiments and field observations; however, there are no records of P. lamellidens workers or brood coexisting in C. obscuripes colonies in field observations, and there is no clear evidence that C. obscuripes is a natural host of P. lamellidens. We conducted detailed field observations, behavioral tests, and rearing experiments to show that C. obscuripes is a host of temporary social parasites. We found colonies with P. lamellidens queens, workers, and larvae intermixed with C. obscuripes workers in the field. Behavioral tests showed that workers of both species in mixed colonies did not attack each other and maintained nestmate recognition ability, which suggests a collaborative nestmate relationship. Furthermore, a rearing experiment confirmed social parasitism by P. lamellidens among C. obscuripes by producing a mixed brood-producing colony. These are the first field and laboratory records of temporary social parasitism involving P. lamellidens and C. obscuripes.