PURPOSE: To enhance the reliability and efficiency of measurements by providing a holding portion for holding a sample, filled with a liquid.

CONSTITUTION: This sample holder 1 is provided with a circular groove (holding portion) H. A sample 3 is set in the groove H, and a fixed amount of liquid 2 of specific gravity is injected to fill the space between the sample 3 and the holder 1. Measuring light 6 is applied to a spot 4 on the portion of the sample 3 which is not immersed in the liquid 2. Because the sample is the same size as the groove H, it does not move inside the groove H, and the absence of a gap leads to improved matching of acoustic impedance, resulting in increased intensity of detection signals and good reproducibility of acoustic propagating conditions, i.e., the holder 1 can easily hold the acoustic contact condition with the solid sample 3 constant, enhancing the reliability and efficiency of measurements. Also, a piezoelectric element 5 for detecting acoustic signals is firmly secured to the holder 1 with an adhesive, etc., and stable acoustic matching between the element 5 and the holder 1 is ensured, thereby further enhancing the reliability of the measurements.