A liquid sample is dropped onto the upper surface of a transparent and cylindrical light-transmitting body (22), and the liquid sample is maintained as a droplet by the surface tension. From above the liquid sample, a transparent cover plate (25) is lowered down to the position where the lower surface thereof touches a spacer (23) in order that the liquid sample is held in the small gap formed between the upper surface of the light-transmitting body (22) and the lower surface of the transparent cover plate (25). A measurement light is provided into the liquid sample held in this manner from immediately above it, and passes through the liquid sample. The transmitted light emitted downwards through the light-transmitting body (22) is introduced into a spectro-detecting unit to be spectro-measured. The measurement optical path length can be adjusted by the height of the spacer (23). This enables an easy transmission spectro-measurement of an extremely small amount of liquid sample.