Joint seal structure of robot, includes fitting end portion provided on an end portion of second member and fittingly inserted into tubular end portion of first member; a peripheral groove provided on an inner peripheral surface of tubular end portion to extend over an entire periphery; and an annular seal member accommodated into peripheral groove and in sealing contact with outer peripheral surface of fitting end portion. Fitting end portion has smaller diameter than body of second member, a stepped surface between body and fitting end portion faces an end surface of tubular end portion and located close to end surface of tubular end portion in an axial direction, and a gap is provided between stepped surface and end surface of tubular end portion such that gap extends over entire periphery, is opened to outside of first member and second member, and is expanded as gap extends radially outward.