A linear motion rolling guide unit, which has underside seals prevented from falling off a slider, comprises a slider having a pair of end caps secured at ends of a casing, and underside seals provided between the pair of end caps having engaging projections. Insertion holes are formed in the underside seals and engaged with the engaging projections to secure the underside seals to the slider. Fallout-inhibiting protrusions are provided at ends of each underside seal in the longitudinal direction. The slider has holding portions each having a hole or a recess into which each of the fallout-inhibiting protrusions is inserted when the underside seal is secured to the slider. A gap is maintained between the holding portion having the hole or the recess and the outer periphery of the fallout-inhibiting protrusion.