A developing device to be attached to and detached from a main body of an electrophotographic image forming apparatus, the developing device comprising: a housing that defines a toner storage area to store toner and a developing portion having a developing roller, wherein the housing further includes an opening that defines a toner supply window to connect the toner storage area to the developing portion, and a communication portion disposed on a sidewall of the housing to electrically connect to the main body when the developing device is mounted on the main body; characterised in that the housing includes a pair of rails adjacent to the toner supply window and extending in an end-to-end lengthwise direction of the developing roller, and an insertion hole on the sidewall of the housing corresponding to a location of the pair of rails wherein a blocking portion of a blocking member is repeatedly insertable in the pair of rails through the insertion hole, and such that the toner supply window is open when the blocking portion is removed from the pair of rails and the toner supply window is closed when the blocking portion is inserted in the pair of rails from an outside of the housing; wherein the blocking member comprises a bent portion extending from the blocking portion and disposed at the outside of the housing when the blocking portion is inserted in the pair of rails, the bent portion being configured to cover the communication portion when the blocking portion is inserted in the pair of rails; and the developing device further comprising a securing portion disposed on the sidewall of the housing to repeatedly secure the bent portion to the sidewall when the bent portion covers the communication portion and so that a predetermined force is required to release the bent portion from the securing portion. a housing that defines a toner storage area to store toner and a developing portion having a developing roller, wherein the housing further includes an opening that defines a toner supply window to connect the toner storage area to the developing portion, and a communication portion disposed on a sidewall of the housing to electrically connect to the main body when the developing device is mounted on the main body; characterised in that the housing includes a pair of rails adjacent to the toner supply window and extending in an end-to-end lengthwise direction of the developing roller, and an insertion hole on the sidewall of the housing corresponding to a location of the pair of rails wherein a blocking portion of a blocking member is repeatedly insertable in the pair of rails through the insertion hole, and such that the toner supply window is open when the blocking portion is removed from the pair of rails and the toner supply window is closed when the blocking portion is inserted in the pair of rails from an outside of the housing; wherein the blocking member comprises a bent portion extending from the blocking portion and disposed at the outside of the housing when the blocking portion is inserted in the pair of rails, the bent portion being configured to cover the communication portion when the blocking portion is inserted in the pair of rails; and the developing device further comprising a securing portion disposed on the sidewall of the housing to repeatedly secure the bent portion to the sidewall when the bent portion covers the communication portion and so that a predetermined force is required to release the bent portion from the securing portion.