A method for observing stem cells by an observation device 1 comprises, placing stem cells C in a petri dish 11, mounting the petri dish 11 on a waveguide 21 via water 13, emitting illumination light L1 into the waveguide 21 and emitting the illumination light L1 to the stem cells C in the petri dish 11 via the water 13, and detecting scattered light L2, the scattered light L2 being the illumination light L1 emitted to the stem cells C that is scattered by the stem cells C and has passed through the waveguide 21. Then, in the light image detected by means of the scattered light L2, a region that is markedly darker than other regions is identified as being in the state tending toward differentiation.