A base plate 20 is provided with an oil inflow hole 20b in the form of a through-hole. The oil inflow hole 20b includes an area overlapping oil inlet communication holes 1 of plates 5 within a region inside an annular grove 20a of the base plate 20, and has a planar shape extending beyond the outer rim of the overlapping area. Thus, in the base plate constituting a contact end when the oil cooler is mounted to the engine block, a decrease in communication resistance of the oil communication hole provided therein as well as a smooth circulation of oil is achieved.