PURPOSE: To simplify the seal structure by providing two airtight spaces to divide clutch operation oil from mission case lubricating oil in a piston part of a clutch device with an oil pressure cylinder to be used for an autobicycle or the like.

CONSTITUTION: An airtight space 48 between the front part of a piston 44 and a partition wall 41 and another airtight space 49 between the partition wall 41 and an oil seal 37 of a cylindrical part 36 are respectively provided. In this construction, when oil pressure is introduced from an oil hole 45, the piston 44 is moved to the right, and a clutch is released via a push rod 31. At this stage, though the lubricating oil in a mission case 13 is sealed by the oil seal 37, the leaked oil there is accumulated in the space 49, and the operation oil leaked by the operation of the piston 44 is accumulated in the space 48 so that they cannot be mixed with each other. Thereby, no other seal is required than the seal 37 for oil sealing.