To improve productivity in forming images on both faces in pitch reference by conducting paper feed scheduling suitably according to a user setting mode, conveyance path parameters, and capacity of image storage memory.

Steps S8 to S11 discriminate the outline of a paper feeding schedule. Namely, when the step 11 discriminates that the number of feeds is zero, it is determined that image output can be made only in the paging order by a reason of small storage capacity, so that the steps 12 to 14 are taken. When the number of feeds is one or more, it is determined that image output can be made in the higher order of efficiency, not in the paging order, so that the steps 15 to 17 are taken. In this case, paper feeding is conducted in the range of the number of IOT output nest times.