To prevent response in a transmission from being lowered and a gear shift shock from occurring, and reduce the cost.

A hydraulic system is comprised of a first structural body having a first oil passage and a valve element housing chamber adjoining to the first oil passage, a second structural element arranged opposed to the first structural body and provided with a second oil passage, a valve element stored in the valve element housing chamber, and a separating member sandwiched between the first and second structural bodies, communicating the first and second oil passages and forming a hole for making a valve seat of the valve element. The valve element is brought in contact with the valve seat when a check valve 35 is not operated so as to shut off the interval between the first and second oil passages. Since the interval between the first and second oil passages is shut-off when the check valve 35 is not operated, it is unnecessary to move the valve element by dynamic hydraulic pressure from the non-operating state into a closed state. Oil at the first oil passage side can be prevented from flowing into the second oil passage.