PURPOSE: To easily measure the thickness of an adhesive layer without destruction when an acoustooptical medium and a piezoelectric vibrator are stuck with an adhesive by strictly measuring the electrostatical capacity of the acoustooptical modulating element.

CONSTITUTION: The acoustooptical modulating element is constituted by providing a lower electrode 4, the piezoelectrical vibrator 2, and an upper electrode 3 on one main surface of the acoustooptic medium 1 in order and joining the piezoelectrical vibrator 2 to the lower electrode 4 with the organic adhesive. Then the acoustooptic modulating element is considered as a capacitor which varies only with the adhesive layer thickness and the relation between the thickness of the adhesive layer 12 of the acoustooptic modulating element and the electrostatic capacity is previously checked. Consequently, the thickness of the adhesive layer 12 can be estimated momentarily by measuring the electrostatic capacity of an element to be measured which is made of the same material in the same shape with the acoustooptical modulating element.