PURPOSE: To create required test conditions very easily and quickly, by changing the spring constant of a buffer body by moving vertically a cylinder body.

CONSTITUTION: A cylindrical cylinder body 5 made of steel is fitted to the outside circumference of a buffer body 4 vertically movably for the buffer body 4. A pair of female screw bodies 6 are attached to position forming 180° in the lower end part of the cylinder body 5. When a stepping motor 8 is driven to rotate a feed screw 7 through gears 9 and 10, the cylinder body 5 moves vertically to change the effective height he of the buffer body 4 (the height of the part of the buffer body 4 exposed from the cylinder body 5), namely, the spring constant of the buffer body 4. Thus, an optional impact acting time is attained.