To optically measure the depth of a liquid in a testing device, which performs measurement by using liquid.

On the lower surface of a measuring instrument 10, a flat surface 22 is provided so as to surround the periphery 23 of the lower surface 22 and a recessed area 25 is formed inside the periphery 23. In the recessed area 25, measuring pads 14 and 16 having various heights are provided as measuring markers. A load is applied to at least one measuring pad 14 with a spring 26 so that the pad 14 may become the same height as that of the contact surface 22 when the measuring instrument 10 is placed on a flat surface.