The invention provides a sheet supply device which can reduce up-and-down movement of pressing paper along with paper supply movement. The sheet supply device comprises a paper supply roller, an input gear, a first output gear, a clutch mechanism, a pressing plate, a cam controlling the ascending displacement and the descending displacement of the pressing plate, a second output gear which meshes with the input gear and rotates to make the cam rotate and consists of a sector gear comprising a first tooth missing part and a second tooth missing part without teeth, a spring applying rotating force to the second output gear, and a maintaining mechanism maintaining a first stop state and a second stop state. In the first stop state, the input gear faces the first tooth missing part, and in the second stop state, the input gear faces the second tooth missing part; when the input gear stops, the first stop state is formed, and when the paper supply roller rotates, the second stop state is formed.