There is provided a reduction gear unit including: a pinion; a bearing that is located on each of both sides of the pinion; a large gear that meshes with the pinion; a gear case that stores therein lubricant oil, and accommodates the pinion and the gear; and pinion bearing caps each of which includes an annular portion, wherein the annular portion extends from outside of the gear case toward inside thereof and parallel to a pinion shaft, and includes an end portion in which an end surface in an extending-direction is provided on a side of the pinion with respect to an extended line of a gear-side end surface of an inner race of the bearing, and an inner peripheral surface of the end portion is formed so that a diameter is decreased from a gear-side end surface of an outer race toward the end surface.