in the liquid chamber 24 to divide the same into first and second liquid chamber sections 24a and 24b the cylinder body 12 is slightly expanded at a portion defining the first liquid chamber section 24a to provide an axially extending groove 24c merged with the section 24a the main piston 28 is formed with a first orifice means 30 which can provide a communication between the first and second liquid chamber sections 24a and 24b as will become clear from the following description the first orifice means 30 is closable by an o ring 32 as is clearly shown in fig3 the main piston 28 is formed with axially spaced first and second land portions 34 and 36 between which a port portion 38 is formed the first land portion 34 is formed with a plurality of equally spaced openings 34a which communicate the first liquid chamber section 24a see fig2 with the port portion 38 and the second land portion 36 is formed with a plurality of axially extending grooves 36a which communicate the port portion 38 with the second liquid chamber section 24b see fig2 thus it will be seen that the openings 34a the port portion 38 and the grooves 36a constitute the above mentioned first orifice means 30 the o ring 32 is loosely coupled in the port portion 38 but it is capable of closing the grooves 36a when brought into contact with the second land portion 36 the main piston 28 is formed with a splined center hole 40 into which a splined base portion 42a of a piston rod.