After a liquid sample is dropped onto a sample-holding platform (11), a window plate (22) is moved closer to the sample-holding platform (11) to hold the liquid sample between them. Measurement light is delivered onto the liquid sample, and the light transmitted through the sample is measured. After the measurement is completed, a window plate holder (23) is raised, and a head (14) is moved from a standby position into the gap between the window plate (22) and the sample-holding platform (11). Then, the window plate holder (23) is lowered so as to press the window plate (22) onto the head (14) until the head (14) touches the sample-holding platform (11) below. From this state, the head (14) is swung back to the original position, whereby a wipe material (40) fitted on the head (14) simultaneously wipes off the liquid sample from both the lower surface of the window plate (22) and the upper surface of the sample-holding platform (11). The sleeve-shaped wipe material (40) is held by a rubber block (33) having wiping surfaces and arch springs (35) on both sides of the rubber block (33). Pressing the arch springs (35) enables the wipe material (40) to be easily attached to or detached from the head (14).