PURPOSE: To enhance heat radiation performance and cooling performance as well, and thereby prevent seizure and the like by forming a groove on each drive clutch plate or each driven clutch plate, and thereby letting cooling oil flow in the aforesaid groove.

CONSTITUTION: Cooling oil is fed to an ahead clutch 6 and an astern clutch 7, and concurrently cooling oil is fed to the inside of a cooling pipe line 22 formed within an input shaft 3. Cooling oil flows in out of the outer circumference of the input shaft 3 at the sections of the advance clutch 6 and the astern clutch 7, and cooling oil is also fed to a drive clutch plate 15, a driven clutch plate 16 and a contact section with a clutch running 17 through hole sections 8b and 9b formed at the cylindrical section 8a of an ahead gear 8 and at the cylindrical section 9a of an astern gear 9, so that cooling oil thereby flows to the outer circumferential side through a groove section 20. In this case, since a cut-out section 21 is formed at the end periphery section of the groove section 20, cooling oil easily flows in the groove section 20, so that the heat radiation of the drive clutch plate is thereby enhanced.