A lock mechanism of a cylinder includes a lock member for locking or
unlocking a piston rod by actions of air pressure and a spring force, a
third pressure chamber for causing air pressure to act on the lock member,
and a third port. In a pneumatic circuit, a check valve and a throttle
valve are connected in parallel in each of first and second pipes
connecting first and second ports of the cylinder and a solenoid valve and
a check valve and a throttle valve are connected in parallel in a third
pipe connecting the third port and the first pipe or the second pipe.
