PURPOSE: To provide the method for a heat treatment which can compensate the oxygen defect in the ferroelectric single crystal.

CONSTITUTION: The ferroelectric single crystal expressed by the chemical formula MTiOXO4 (M denotes one element selected from a group consisting of K, Rb and Ti; X denotes one element selected from a group consisting of P and As) is heat treated at a Curie temp. or below and in an atmosphere contg. oxygen components. As a result, the oxygen deficiency in the single crystal is compensated and the photodamage resistant characteristic is improved. The photodamage resistant characteristic and generation efficiency of SHG are greatly improved particularly by confining the total content of the bivalent and tervalent Ti to ≤20% of the total weight of the Ti in the single crystal.