A linear motion rolling guide unit with underside seals not falling away from a slider even when mounting the slider on a track rail. An engaging member comprises a protrusion having two faces extending parallel to the sliding direction of the slider, one facing the track rail and the other facing away the track rail. The protrusion comprises an inner hook protruding toward the track rail and an outer hook protruding in the opposite direction and being out of phase with the inner hook in the sliding direction of the slider, and moves through the fitting hole while being elastically deformed in the process of pressing the engaging member into the fitting hole after the inner hook has been engaged with the fitting hole, and returns to its original shape when the outer hook passes through the fitting hole, whereby the inner and outer hooks hold the underside seal.