Disclosed is an observation device which can acquire a phase image of a moving object at high speed and high accuracy even when using a photodetector in which the readout speed per pixel is low. An observation device (1) is provided with a light supply unit (10), a first modulator (20), a second modulator (30), a lens (40), a beam splitter (41), a photodetector (46), and a computation unit (50). The lens (40) inputs scattered light generated by a moving object (2) and generates a Fourier transform of the object (2). The photodetector (46) outputs data at each time point about each position in a u-direction, said data showing the sum of a v-direction of data which changes temporally at a frequency corresponding to the Doppler shift amount of the light which reaches each position of a light receiving surface across the lens (40). The computation unit (50) acquires an image of the subject (2) on the basis of the output of the photodetector (46).