PURPOSE: To simplify the constitution by connecting an actuator to a point of force provided between clamping mechanisms and thereby enabling the chucking of tested pieces, etc. and the clamping of each clamping mechanism to each strut with the single actuator.

CONSTITUTION: When the oil pressure is supplied to the port 8 of an oil-hydraulic jack 6, a plunger 7 is pressed down and a piece 4 to be tested is chuked by a part 5 of a chuck 3. The counter-force to the force pressing down the part 5 of the chuck 3 acts upward on the jack 6 and pin joints 14 and 15, perpendicularly to the clamping direction of the clamp shoes 12 and 13 of clamping mechanisms 10 and 11. Accordingly, the shoes 12 and 13 are drawn inward by the component force in the horizontal direction of the counterforce transmitted by pins 16 and 17, thus clamping each of struts 2 and 2'. As the result, the piece 4 to be tested is chucked and simultaneously a cross-head 1 is fixed to the struts 2 and 2'.