To provide an image processing apparatus capable of outputting image data not rotated and rotated image data simultaneously and effectively using storage area of a memory.

Partial image data with a prescribed number of pixels being part of image data for one page stored in an original buffer area 10a of a page memory 10 are written in a row direction to a buffer and thereafter the data are read from the buffer to the page memory 10 in a column direction to carry out transposition processing and transposed partial image data after the transposition processing are written in a first transposition buffer area 10b and second transposition buffer area 10c of the page memory 10. The processing of reading original image data from the original buffer area 10a and the processing of reading the transposed partial image data from the first transposition buffer area 10b and the second transposition buffer area 10c are carried out simultaneously. The first transposition buffer area 10b and the second transposition buffer area 10c with a required minimum storage capacity are secured in succession to the original buffer area 10a.