To provide a method for magnetization by which a Bi-replaced rare- earth-iron-garnet film, etc., can be magnetized surely in a single magnetic domain without requiring any large-scale magnetic-field generating device.

In this method of magnetizing Bi-replaced rare-earth-iron-garnet film, the Bi-replaced rare-earth-iron-garnet film 1 which is a high-coercive force film maintained in a magnetically saturated state, even when no external magnetic field exists is cooled after the film 1 is heated to a temperature of ≥50°C lower than the curie temperature of a permanent magnet 2, while a magnetic field of ≥15.92 kA/m is impressed upon the film 1 by means of the magnet 2 in a state where the film 1 is kept on an object or incorporated in an optical isolator chip (magnetooptical device) 10, etc.