We developed a small wireless device based on a lensless CMOS imaging system that enables observing the neural activities in the mouse brain. The weight of the device was 4.6 g and the dimensions were 15 × 24 mm2 with a Bluetooth low-energy (BLE) module and a microcontroller embedded analog-to-digital (ADC) function. This device is driven by a 3-V lithium coin battery. This system has successfully transferred the image at a frame rate of 0.95 fps with a 16×30-pixel size. The result suggests that the device could be applied for in-vivo experiments in a freely moving mouse.