PURPOSE: To measure the coefficient of linear expansion of a soft material by projecting a laser beam on a sample and receiving the light reaching to a photoelectric element without being intercepted by the sample by the element, and then, finding the sample scanning time from the output of the element.

CONSTITUTION: The laser beam of a laser light source 1 scans a sample 8 arranged perpendicularly to the optical axis of an fθ lens 4 after it is reflected by a polygon mirror 6 rotated by a motor 5 and passed through the lens 4. Since the scanning beam is intercepted by the sample 8 while the beam scans the sample 8, the level of the signals received by a photoelectric element 11 through a condenser lens 10 drops. Pulse signals synchronous to the scan starting and terminating timing of the sample are obtained from the signals through an amplifier 12 and edge detection circuit 13. In order to measure the time interval of the pulse signals, the output clock of a clock oscillator 15 is counted by a counter 17 and the count value is fetched to a microcomputer 21 for calculating the scanning time. When the scanning time is calculated, the length of the sample 8 is obtained. Then the temperature is changed and the length of the sample 8 is again measured similarly. Therefore, the coefficient of linear expansion of the sample 8 can be measured from a change in length of the sample before and after the temperature change.