A paper size discriminator of the size of a stacked printing paper includes a pair of members which are relatively displaced in the direction of the width of the loaded paper. A long detecting plate is attached to one of the members. The detecting plate includes a grove that is progressively stepped in a direction transverse to the width of the paper. A pin sliding in the groove is attached to the other member such that the size of loaded paper is ranked in the direction of the width by detecting the displacement of the pin. The rear end keep plate is displaced corresponding to the length of paper wherein the length of the paper is discriminated by combining output signals from first and second detecting switches.