Phase-locking of two longitudinal oscillating modes of a dual-mode microchip laser that has 101 GHz longitudinal-mode spacing has been successfully demonstrated by using electronic negative-feedback control. Since the two longitudinal modes were oscillating simultaneously in the same laser cavity, the common-mode fluctuation cancels, resulted in the small required control bandwidth of ∼4 kHz. This narrow-bandwidth phase-locking technique is useful for generating high-spectral-purity millimetre-wave signals with a considerably simpler setup.