A detection device for detecting a state of a waste toner container comprises an optical sensor and an actuation member. The optical sensor comprises an emitting unit emitting an optical signal and a receiving unit receiving the optical signal. A space is provided between the emitting and receiving units, and the optical signal travels along an optical path from the emitting unit to the receiving unit. The actuation member is movably disposed between the emitting and receiving units. The actuation member in a first position interrupts the optical path of the optical signal; and the actuation member is moved by the waste toner container to a second position when at least a part of the waste toner container is placed in the space. The actuation member in the second position allows a passage of the optical signal. An image forming apparatus using such a detection device is also provided.