The second housing has a side panel extending in the mating direction and at least three second connector lock portions protruding from the outer surface of the side panel, the second connectors each being arranged in the transverse direction of the second housing and separated by an interval, and the first housing has a side panel extending in the mounting direction, a locking arm portion formed in the side panel and covering at least a portion of the outer surface of the side panel of the second housing when the first housing and the second housing are mated, and at least three first connector locking portions each formed in the locking arm portion in the transverse direction of the first housing and separated by an interval, and each engaging a second connector locking portion when the first housing and the second housing are mated.