A ferromagnetic resonator is disclosed, in which perpendicular ferrimagnetic resonance of thin film YIG is utilized. The resonator comprises a ferrimagnetic YIG thin film and a microstrip line coupled to the YIG thin film operative in a bias magnetic field applied perpendicular to a major surface of the YIG thin film. The YIG thin film disk has a magnetization distribution of magnetostatic mode, and the microstrip line is designed to generate a high-frequency magnetic field distribution similar to the magnetization distribution of the uniform mode (1, 1).sub.1. In such arrangement, coupling between the high-frequency magnetic field and magnetization of high sprious mode is reduced.