PURPOSE: To exactly monitor the progress state of development by irradiating a laser beam enlarging a beam diameter to an optical master disk and processing arithmetically the light quantity signal of 0th-order and ±1st-order diffracted beams.

CONSTITUTION: The laser beam 3 enlarging the beam diameter by a beam diameter adjusting part 4 is made incident on the pattern face of the optical master disk 1, a development ending signal (d) is outputted by processing arithmetically with an arithmetic part 11 the light quantity signals (a), (b) and (c) converting photoelectrically and amplifying a signal attained 0th-order and ±1st-order diffracted beams 5, 6 and 7 by sensors 8, 9 and 10. Thus, development monitoring without being influenced by developer flows on the pattern surface is performed.