A sheet folding device comprising: forming belts which are disposed on both sides in a transfer direction (D) of a corrugated board (S) and move to a center side in a width direction of the corrugated board (S) toward a downstream side in the transfer direction (D) of the corrugated board (S) to press both end portions in the width direction of the corrugated board (S) from an outside so as to bend both the end portions; and a gauge roller group including several gauge rollers which are disposed along the transfer direction (D) of the corrugated board (S) and come into contact with outer sides of both bent portions in the width direction of the corrugated board (S) bent by the forming belts , wherein in the several gauge rollers , recessed portions recessed toward a rotation axis side in a radial direction are provided on outer peripheral surfaces of the gauge rollers in a circumferential direction, and positions of the recessed portions in a direction of the rotation axis (O) at deepest positions (P) are disposed to be offset downward in a vertical direction toward a downstream side in the transfer direction (D) of the corrugated board (S), each of the recessed portions has an upper support surface , a lower support surface , and a deepest support surface which connects the upper support surface and the lower support surface to each other, and the deepest support surface has a curved surface shape or a planar shape, the deepest positions (P) of the recessed portion are a position of the recessed portion closest to a rotation axis (O) of the gauge rollers , characterized in that the sheet folding device further comprising: a gauge roller adjustment device configured for moving the gauge roller group in parallel in a thickness direction of the corrugated board (S) and adjusting a position of the gauge roller group in the direction of the rotation axis (O). forming belts which are disposed on both sides in a transfer direction (D) of a corrugated board (S) and move to a center side in a width direction of the corrugated board (S) toward a downstream side in the transfer direction (D) of the corrugated board (S) to press both end portions in the width direction of the corrugated board (S) from an outside so as to bend both the end portions; and a gauge roller group including several gauge rollers which are disposed along the transfer direction (D) of the corrugated board (S) and come into contact with outer sides of both bent portions in the width direction of the corrugated board (S) bent by the forming belts , wherein in the several gauge rollers , recessed portions recessed toward a rotation axis side in a radial direction are provided on outer peripheral surfaces of the gauge rollers in a circumferential direction, and positions of the recessed portions in a direction of the rotation axis (O) at deepest positions (P) are disposed to be offset downward in a vertical direction toward a downstream side in the transfer direction (D) of the corrugated board (S), each of the recessed portions has an upper support surface , a lower support surface , and a deepest support surface which connects the upper support surface and the lower support surface to each other, and the deepest support surface has a curved surface shape or a planar shape, the deepest positions (P) of the recessed portion are a position of the recessed portion closest to a rotation axis (O) of the gauge rollers , characterized in that the sheet folding device further comprising: a gauge roller adjustment device configured for moving the gauge roller group in parallel in a thickness direction of the corrugated board (S) and adjusting a position of the gauge roller group in the direction of the rotation axis (O). <EOS>