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; wherein the cassette mounting unit includes: an insertion and removal opening into or from which the cassette is inserted or removed; a rotation drum having a plurality of holding spaces for the cassettes radially; and a strut through which the rotation drum is inserted; characterized in that the image acquisition comprises: a distance sensor that measures a distance to the cassette mounted in the cassette mounting unit and outputs a detection signal based on a measurement result; and an information generation unit that generates state information on a mounting state of the cassette mounted in the cassette mounting unit on the basis of the detection signal; wherein: the cassette mounting unit further includes a back opening located on the side opposite to the insertion and removal opening the distance sensor is a reflective distance sensor that measures a distance to a back surface portion of the cassette through the back opening; the image acquisition device further comprises an operation control unit that controls an operation of the rotation drum on the basis of the state information generated by the information generation unit; and the distance sensor is provided in the strut via a support member. 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; wherein the cassette mounting unit includes: an insertion and removal opening into or from which the cassette is inserted or removed; a rotation drum having a plurality of holding spaces for the cassettes radially; and a strut through which the rotation drum is inserted; characterized in that the image acquisition comprises: a distance sensor that measures a distance to the cassette mounted in the cassette mounting unit and outputs a detection signal based on a measurement result; and an information generation unit that generates state information on a mounting state of the cassette mounted in the cassette mounting unit on the basis of the detection signal; a distance sensor that measures a distance to the cassette mounted in the cassette mounting unit and outputs a detection signal based on a measurement result; and an information generation unit that generates state information on a mounting state of the cassette mounted in the cassette mounting unit on the basis of the detection signal; wherein: the cassette mounting unit further includes a back opening located on the side opposite to the insertion and removal opening the distance sensor is a reflective distance sensor that measures a distance to a back surface portion of the cassette through the back opening; the image acquisition device further comprises an operation control unit that controls an operation of the rotation drum on the basis of the state information generated by the information generation unit; and the distance sensor is provided in the strut via a support member. the cassette mounting unit further includes a back opening located on the side opposite to the insertion and removal opening the distance sensor is a reflective distance sensor that measures a distance to a back surface portion of the cassette through the back opening; the image acquisition device further comprises an operation control unit that controls an operation of the rotation drum on the basis of the state information generated by the information generation unit; and the distance sensor is provided in the strut via a support member.