An information recording/reproducing device is provided with a tray (3) having holders (2, 4) for holding a disc (9) on a disc placing part (11) by permitting the disc to overhang on a part of the disc placing part (11) in a vertically laid status. The information recording/reproducing device is provided with a means for detecting an abnormally mounted status, wherein the disc (9) is placed on the disc placing part (11) through the holding holders (2, 4), by comparing a number of rotations or load torque measured at a prescribed time after a rotating means (8) is started, a measured time required for the number of rotations to reach a fixed number of rotations or a drive current measured when the number of rotations became the prescribed umber of rotations, with a previously stored threshold value. Thus, detection of the abnormally mounted status can be completed for the disc prior to starting recording or reproducing operation.