PURPOSE: To measure the size of micrograin accurately without reducing the spot diameter, by scanning the specimen surface at a smaller scanning pitch than the spot diameter of scanning light, and superposing and storing the scattered intensity signals obtained by plural scanning lines.

CONSTITUTION: The A/D converted value of scattered light intensity obtained by the first scanning line enters a register 31, and the A/D converted value by the second scanning line enters a register 32. The outputs of respective bits of these registers 31 and 32 are applied to an adder 34. Addition of these values is controlled by the periodic signal generated at every scanning line. Therefore, an adder circuit 34 operates with the periodic signal generated when the scanning of the third line is over. At this time, the contents stored in the registers 31 and 32 are added in the corresponding bits. The result of additon is stored in a line memory 35. Therefore, whenever one scanning is finished, the contents of sum of two scannings is always stored in the line memory 35.