PURPOSE: To make it possible to easily find out optimum conditions and to obtain a free electro-optic constant by controlling production conditions while monitoring the electro-optic constant of nonlinear molecules.

CONSTITUTION: An electrode part is formed with multilayered reflection films of dielectric substances on a substrate and has electrodes thereon, which electrodes are coated with the electro-optic material. This electrode part is mounted at the inside of a temp. controller 10 and the electro-optic material is irradiated with a laser beam from a laser beam source 5. The electro-optic material is so composed that the voltage from a control power source 11 is impressed between the electrodes. The laser beam reflected from the electro- optic material changed by the impressed electric field is detected by a photodiode 16. The detection signal is admitted from a signal input terminal 18 to a lock-in amplifier 17 and the lock-in is detected by a reference signal 13. The magnitude of the detection signal reflects the magnitude of the electro-optic effect and, therefore, the electric field intensity to be impressed between the electrodes us controlled by a control power source 11 while observing there-of.