A linear motion rolling guide unit capable of being adapted to an ultra-reduction in size. A recessed portion 23 is provided in each of the side faces 18 of a track rail R below a lower raceway face 2 and extends parallel to the axis of the track rail R. An underside seal 24 has a seal portion 24d facing the recessed portion 23. Engaging hooks 24c are provided at the two ends of the underside seal 24. End caps 3, 4 have engaging holes 26 engaging with the respective engaging hooks 24c. The underside seals 24 and the end caps 3 and 4 are each formed of a synthetic resin in one piece.