An image forming apparatus has a sheet feeding portion feeding a sheet; a conveyance passage for conveying the sheet inside the apparatus; an image forming portion forming an image on the sheet; a skewing detector disposed in the conveyance passage between the sheet feeding portion and the image forming portion, detecting arrival and departure of the sheet, and detecting skewing amount of the sheet in the conveyance passage; a display portion displaying an apparatus state; a storage portion storing data; and a control portion controlling sheet feeding and conveying, and grasping the sheet skewing amount based on the output of the skewing detector to stop printing when the skewing amount is at or above a first predetermined amount stored in the storage portion.