In a lubrication structure of a differential gear unit, a case has a guide rib that extends over a certain area with respect to a rotation path of a head of a bolt when a ring gear rotates, and that protrudes out from the case toward the head of the bolt. Also, a through-hole that extends in a vertical direction is formed in an upper portion of a hollow supporting portion, and the guide rib has an oil reservoir that curves toward an upper end opening of the through-hole.