We applied scanning internal photoemission microscopy (SIPM) to characterize the degradation of GaN Schottky contacts formed on a thick n-GaN layer grown on a freestanding GaN substrate by in situ applying reverse bias voltage (Vbias) down to −45 V. For most of the contacts, uniform distribution of the photocurrent was observed over the electrode with the visible lasers. Irregular-shape regions with 5%-25% larger photocurrent appeared with the near UV laser by applying Vbias, but the I-V characteristics were stable. On the other hand, for the contacts with a slightly larger reverse current, the photocurrent distribution was also uniform at Vbias = 0 V, but over Vbias = −36 V, the photocurrent was intensively increased at small spots. After the SIPM measurements, the I-V characteristics became leaky, and the same spots were observed in the microscope image. These results indicate that SIPM is useful for in situ monitoring of the initial stage of the degradation under applying reverse bias voltage.