passages formed in the crankcase 12 as shown in fig4 a cooling unit 51 is assembled to the crankcase 12 of the engine 31 more specifically as shown in fig2 and fig3 the cooling unit 51 is assembled to the front side of the crankcase 12 in the vehicle running direction f the cooling unit 51 forms an oil cooler 52 on which an oil filter 53 is removably mounted as shown in fig6 the engine oil flowing into the oil pump 48 through a strainer 54 is supplied to the cooling unit 51 after the engine oil 14 is cooled in the oil cooler 52 the engine oil 14 is filtered by a filter element 53 a in the oil filter 53 so as to be supplied to oil requiring portions the cooling unit 51 comprises a pair of half bodies such as a base plate 55 and an overlapping plate 56 the base plate 55 is formed approximately rectangular shape an inner surface of the base plate 55 is defined as a flat matching surface 55 a as shown in fig7 on the other hand the flat portion is formed in the half area of the outside surface of the base plate 55 to define a mounting surface 55 b to the crankcase 12 as shown in fig8 the shape of overlapping.