A developing device includes a toner storage area containing toner, a developing portion in which a developing roller is installed, and a housing including a toner supply window to connect the toner storage area and the developing portion. A pair of rails is disposed on the toner supply window to extend in a lengthwise direction of the developing roller, and a blocking member including a blocking portion is provided to open and close the toner supply window when the blocking member is removed from and inserted into the pair of rails from outside of the housing, respectively.