1. A biological image acquisition device comprising: a specimen support for supporting a biological specimen placed thereon; a two-dimensional detector; an image pickup optical system for leading, to the two-dimensional detector, light traveling in a direction toward the two-dimensional detector from the specimen placed on the specimen support and light traveling in another direction from the specimen, and for forming images of these lights on the two-dimensional detector, the image pickup optical system comprising a reflection mirror which leads the light traveling in the other direction than the direction toward the two-dimensional detector directly to the two-dimensional detector from the reflection mirror; a light source system comprising a light source arranged outside a field of view of the two-dimensional detector, the light source irradiating the specimen placed on the specimen support with light and irradiating at least a part of the reflection mirror with direct light, and the field of view comprising indirect field of view of the two-dimensional detector through the reflection mirror and direct field of view of the two-dimensional detector; and an image display device for displaying images picked up by the two-dimensional detector, wherein no light source is arranged within a field of view of the two-dimensional detector.