A developing device includes: a housing having an opening; a sheet member provided on the housing and sealing the opening when not used, the sheet member including a forward part and a backward part, the forward part extending from a base end to a folded part, and the backward part extending in the one direction from the folded part to a free end as a top end of the sheet member; and a press unit pressing the backward part of the sheet member toward the forward part when not used, wherein the housing has a guide hole formed in a position where the backward part gradually separates off from the forward part and from the folded part to the free end, the backward part being inserted into the guide hole when not used, and the sheet member being guided by the guide hole when the sheet member is pulled out.