PURPOSE: To make it possible to detect the fall of the frost in advance, by decreasing the temperature of a reflecting plate faster than that of an object to be measured, and detecting the fall of the frost on the reflecting plate as the variation in reflectivity of light.

CONSTITUTION: In response to the decrease in atmospheric temperature at winter night and the like, the temperatures of tea plants 1 and the reflecting plate 7 also decrease. The specific heat of the plate 7 is smaller than that of the plant 1, and the temperature of the plate 7 decreases faser than that of the plant. Whent it is frosted, the reflectivity of the plate varies from the low value to the high value. The output of a light receiving device 8 becomes large. When a reference value is exceeded, a display 10 is driven, and the fall of the frost on the plant 1 can be detected in advance.