A step unit that guides a sliding door of a vehicle includes a step member and a rail plate member. The step member includes first and second lower rails. The first lower rail has two ends that face toward each other to form a cutout portion in between. The rail plate member is arranged in the cutout portion. The step member includes an insertion hole and an extended portion. The insertion hole is located at a position corresponding to the cutout portion. The extended portion is extended from at least one of the two ends to support the rail plate member against a load received by the rail plate member from the roller. The rail plate member is inserted through the insertion hole from above the upper side of the step member and fixed by a fastening member to the step member.