We have developedultrathin MEMS mirror devices. The mirror device was driven by lead zirconate titanate (PZT) actuators. Because the thickness of the mirror device is 5.31 µm, the mirror device is flexible. The mirror device was mounted on a polyimide substrate by a mounting method that we have developed. After fabrication, we investigated the characteristics of the mirror device experimentally. The frequency of the resonant mode for scanning optical light was 10.4 kHz. Also, we conducted an experiment to confirm the flexibility of the mirror device. When the polyimide substrate was bent (radius of curvature was 125 mm), the mirror device can be actuated without breakage. Moreover, it was observed that the resonance frequency and mechanical angle of the mirror device were dependent on the radius of curvature of the flexible substrate. This fabrication and mounting technology is highly promising for the development of flexible hybrid electronic (FHE) devices.