To form a crease with high quality using a low-cost mechanism without causing damage such as a wrinkle or a scratch on a sheet member or a reduction in the productivity.

A sheet folding device includes upper and lower pressing plates 219 and 220 that have convexly curved surfaces to face each other, and that form the crease on the leading end part P1 of the paper sheet bundle PB by holding and pressing the paper sheet bundle PB therebetween, which has been pushed into a folding part, and in the standby state, an upper pressing unit 217 and a lower pressing unit 218 are separated from each other by press-releasing cams 209 and 210 provided at the inside of a pressing moving plate 208, when the moving plate is driven, the press-releasing cams 209 and 210 connected to the moving plate are moved in the same direction, and rollers 213 and 215 of the upper pressing unit 217 and rollers 214 and 216 of the lower pressing unit 218 are moved in a direction to come close to one another along the inclined surfaces of the press-releasing cams 209 and 210, then, press the leading end P1 of the paper sheet bundle PB.