A sliding type constant velocity universal joint is provided that can prevent a pressure buildup inside the joint when fitting a boot adapter to an outer joint member. The outer joint member has a seal portion in an outer surface on the far end side of a portion that mates with a fitting portion of the boot adaptor. The seal portion is fitted with the fitting portion in an airtight manner. A small diameter portion having a smaller diameter than the seal portion is provided in a region nearer to an open end of the outer joint member than the seal portion to form a gap between the small diameter portion of the outer joint member and the fitting portion of the boot adaptor.