1. An observation image determination device comprising: a memory; an imaging device which captures an observation image; and a processor coupled to the memory and the processor configured to: capture an image of an observation region including a stem cell to be cultured to acquire the observation image; and determine whether a living body of a different type from the stem cell is included in the observation region, wherein whether the different type of living body is included in the observation region is determined 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, wherein the spatial frequency characteristics is a spectrum of a spatial frequency component, and wherein the imaging device is adjusted to determine whether the different types of living body is included in the captured observation image.