A waste toner collecting device includes a box that stores a waste toner unnecessary in image formation, a collection port that is provided in an upper part of a main body of the box, receives the waste toner, and drops the waste toner into the box, a triangular obstacle provided in a position opposed to a falling port for the waste toner and near a bottom surface in the box, and a sensor that is provided in a position lower than the collection port and higher than the obstacle and detects that an amount of the waste toner stored in the box reaches a predetermined amount.