Provided is an observation device which can obtain a phase image of a moving object rapidly with high sensitivity even when using a photodetector having a slow read-out speed per pixel. The observation device 1 comprises a light source 10, a first modulator 20, a second modulator 30, a lens 40, a beam splitter 41, a photodetector 46, and an arithmetic unit 50. The lens 40 receives scattered light generated by a moving object 2 and forms a Fourier transform image of the object 2. The photodetector 46 outputs data representing a sum in a v direction of data temporally changing at a frequency corresponding to a Doppler shift frequency of the light having reached each position on a light-receiving surface through the lens 40 at each position in a u direction at each time. The arithmetic unit 50 obtains an image of the object 2 according to the output of the photodetector 46.