A corrugated sheet folding device comprising: forming belts which are disposed on both sides in a transfer direction of a corrugated fiberboard and are configured to move to a center side in a width direction, which is orthogonal to the transfer direction, of the corrugated fiberboard toward a downstream side in the transfer direction so as to press and bend both end portions of the corrugated fiberboard in the width direction from outside; and forming rollers (123, 124, 125, 126) which are disposed on the center side in the width direction from the forming belts on both sides in the transfer direction and are configured to come into contact with inner sides of both bending portions of the corrugated fiberboard in the width direction, wherein the forming rollers (123, 124, 125, 126) are disposed on an upstream side in the transfer direction from a 90° bending position of the corrugated fiberboard, wherein the forming roller (123, 124, 125, 126) includes a protrusion portion (123a, 124a, 125a, 126a) formed by an intermediate portion in the width direction protruding outward in a radial direction, an inner peripheral surface (123b, 124b, 125b, 126b) which is provided on a center side in the width direction from the protrusion portion (123a, 124a, 125a, 126a), and an outer peripheral surface (123c, 124c, 125c, 126c) which is provided on an end portion side in the width direction from the protrusion portion (123a, 124a, 125a, 126a), characterized in that an angle of the inner peripheral surface (123b, 124b, 125b, 126b) with respect to an axial direction is larger than an angle of the outer peripheral surface (123c, 124c, 125c, 126c) with respect to the axial direction. forming belts which are disposed on both sides in a transfer direction of a corrugated fiberboard and are configured to move to a center side in a width direction, which is orthogonal to the transfer direction, of the corrugated fiberboard toward a downstream side in the transfer direction so as to press and bend both end portions of the corrugated fiberboard in the width direction from outside; and forming rollers (123, 124, 125, 126) which are disposed on the center side in the width direction from the forming belts on both sides in the transfer direction and are configured to come into contact with inner sides of both bending portions of the corrugated fiberboard in the width direction, wherein the forming rollers (123, 124, 125, 126) are disposed on an upstream side in the transfer direction from a 90° bending position of the corrugated fiberboard, wherein the forming roller (123, 124, 125, 126) includes a protrusion portion (123a, 124a, 125a, 126a) formed by an intermediate portion in the width direction protruding outward in a radial direction, an inner peripheral surface (123b, 124b, 125b, 126b) which is provided on a center side in the width direction from the protrusion portion (123a, 124a, 125a, 126a), and an outer peripheral surface (123c, 124c, 125c, 126c) which is provided on an end portion side in the width direction from the protrusion portion (123a, 124a, 125a, 126a), a protrusion portion (123a, 124a, 125a, 126a) formed by an intermediate portion in the width direction protruding outward in a radial direction, an inner peripheral surface (123b, 124b, 125b, 126b) which is provided on a center side in the width direction from the protrusion portion (123a, 124a, 125a, 126a), and an outer peripheral surface (123c, 124c, 125c, 126c) which is provided on an end portion side in the width direction from the protrusion portion (123a, 124a, 125a, 126a), characterized in that an angle of the inner peripheral surface (123b, 124b, 125b, 126b) with respect to an axial direction is larger than an angle of the outer peripheral surface (123c, 124c, 125c, 126c) with respect to the axial direction.