PURPOSE: To automate the measurement, increase the measuring precision, and shorten the time, by disposing a thermometer in front of surface light source, and detecting the liquid column height at the opposing position by means of an image pickup device.

CONSTITUTION: A medical thermometer 2 is disposed in the central position of a surface light source 1 provided in a thermostatic chamber, and, opposedly to the thermometer 2, an image pickup device 4 using CCD, etc., is disposed in front of the surface light source. Predetermining the range Ls detectable by the image pickup element 4 having the length of ls, and the height h from the reference plane A of the upper limit position of the detecting range, when measuring, the length L0 from the upper end of the mercury column up to the upper limit of the detecting range Ls is detected according to the output from the element 4, and (h-L0) is calculated. Thus, the height hx of the mercury column from the reference plane A can be detected.