An optical measurement device and an optical measurement method for measuring a gap on an electronic device are provided. The optical measurement device includes an image capture device and an image processing device. The image capture device is used to capture an image including a gap. The electronic device includes a body and a component assembled on the body. The gap of the electronic device is existed between the body and the component. The image processing device is coupled to the image capture device. The image processing device is used to receive the image, and the image scanning operation is executed to the image to measure the breadth of the gap. The precise measurement result is quickly obtained.