An image acquisition device for acquiring an image of samples held on slide glasses, the image acquisition device comprising: a cassette mounting unit in which a cassette is detachably mounted, the cassette holding the slide glasses in a plurality of stages in a predetermined arrangement direction, wherein the cassette mounting unit comprises a rotation drum including a bottom plate, a top plate, a plurality of cassette holding plates disposed between the bottom plate and the top plate to define a plurality of holding spaces, and a strut serving as a non-rotating shaft and provided between a central portion of the top plate and a central portion of the bottom plate; a light source configured to emit inspection light toward the cassette mounted in the cassette mounting unit; a scanning unit configured to perform scanning with the inspection light in the arrangement direction; a light reflection unit that is disposed on a side surface of the strut to face the light source and configured to reflect the inspection light emitted from the light source; a light detection unit configured to detect reflected light including the inspection light reflected by at least one of the light reflection unit, the cassette, and the slide glass, and configured to output a detection signal; and an information generation unit configured to generate holding information on at least one of a holding position and a holding state of the slide glass held in the cassette on the basis of the detection signal, wherein the rotation drum is rotatable around the strut, and can be stopped at a position at which a holding space is located between the light source and the light reflection unit. a cassette mounting unit in which a cassette is detachably mounted, the cassette holding the slide glasses in a plurality of stages in a predetermined arrangement direction, wherein the cassette mounting unit comprises a rotation drum including a bottom plate, a top plate, a plurality of cassette holding plates disposed between the bottom plate and the top plate to define a plurality of holding spaces, and a strut serving as a non-rotating shaft and provided between a central portion of the top plate and a central portion of the bottom plate; a light source configured to emit inspection light toward the cassette mounted in the cassette mounting unit; a scanning unit configured to perform scanning with the inspection light in the arrangement direction; a light reflection unit that is disposed on a side surface of the strut to face the light source and configured to reflect the inspection light emitted from the light source; a light detection unit configured to detect reflected light including the inspection light reflected by at least one of the light reflection unit, the cassette, and the slide glass, and configured to output a detection signal; and an information generation unit configured to generate holding information on at least one of a holding position and a holding state of the slide glass held in the cassette on the basis of the detection signal, wherein the rotation drum is rotatable around the strut, and can be stopped at a position at which a holding space is located between the light source and the light reflection unit.