1. An observation image determination device comprising: an observation image acquisition unit that captures an image of an observation region including a stem cell to be cultured to acquire an observation image; and a determination unit that determines whether a living body of a different type from the stem cell is included in the observation region, wherein the determination unit determines whether the different type of living body is included in the observation region on the basis of spatial frequency characteristics of an observation target acquired from the observation image and predetermined spatial frequency characteristics of the different type of living body.