An area calculating unit calculates an image area of a toner image formed on a sheet recording member. A behavior detecting unit detects behavior of the recording member fed out of a conveying nip formed by a pair of conveying members. A thickness obtaining unit obtains thickness information of the recording member. An index calculating unit calculates an index value indicating windability of the recording member with respect to the conveying members based on the image area, the behavior, and the thickness information.