A clutch can comprise driving clutch plates spline-fitted on an input member, driven clutch plates arranged alternately with the driving clutch plates and spline-fitted on an output member, a clutch piston having one surface facing an oil pressure chamber and another surface facing the clutch plates and adapted to be displaced toward the plates by supplying oil to the oil pressure chamber and to be returned by discharging the oil from the chamber. A valve can pass through the clutch piston and be adapted to be closed for preventing oil from exiting the chamber to the other side of the clutch piston when the clutch is clutched-on and to be opened and allow airflow to the oil pressure chamber when the clutch is clutched-off. One or more weight reduction recesses can be formed on one surface of the clutch piston near the outer circumference further radially outward than the valve.