PURPOSE: To reduce cost and weight by arranging open/close valves and pressure detecting means between pressure adjusting means and elastic shrinking bodies, and computing shrinkage amounts of the elastic shrinking bodies according to the pressure values upon the closure of the open/close valves.

CONSTITUTION: Open/close valves 6a, 6b and pressure detecting means 7a, 7b are arranged between pressure control means 4a, 4b and elastic shrinking bodies 1a, 1b. Signals from the pressure detecting means 7a, 7b are input into a computing means 9 and shrinkage amounts of the elastic shrinking bodies 1a, 1b are computed according to pressure values therein upon the closure of the open/close valves 6a, 6b. It is unnecessary, therefore, to use a comparatively expensive displacement detector, so that cost can be reduced.