Image forming apparatus capable of adjusting print position of image on recording paper

An image forming apparatus, comprising:

• a plurality of paper feed units;
• a print unit for forming an image and printing the formed image on a recording paper;
• a registration roller, arranged between the paper feed units and the print unit, for receiving a recording paper transported from one of the paper feed units and transporting the recording paper to the print unit;
• a sheet leading end sensor provided downstream, in a transport direction of the recording paper, of the registration roller, for detecting a leading end of the recording paper;
• a time measuring unit for measuring a time from a start point of transportation of the recording paper by the registration roller to a point of detection of the leading end of the recording paper by the sheet leading end sensor;
• a memory unit for storing a reference image forming time, which is a reference value of a length of an image forming period from an image formation start point in the print unit to the start point of transportation of the recording paper by the registration roller and a reference time, which is a reference value of a time from the start point of transportation to the point of detection of the leading end of the recording paper by the sheet leading end sensor;
• a print control unit for adjusting, when a recording paper is fed from an arbitrary paper feed unit among the paper feed units, a print position of an image on the recording paper by controlling the print unit and the registration roller to adjust the length of the image forming period;
• and
• an arithmetic unit for obtaining, as a comparative time, a time detected by the time measuring unit when the leading end of the recording paper is detected by the sheet leading end sensor, obtaining a time difference between the obtained comparative time and the reference time stored in the memory unit every time a recording paper is fed from the paper feed unit, and storing the time difference in the memory unit in association with the paper feed unit from which the recording paper is fed,
• wherein the print control unit adjusts, when the recording paper is fed from a specific paper feed unit among the paper feed units, the length of the image forming period in accordance with an adjusted image forming time obtained by subtracting the time difference, previously obtained by the arithmetic unit and stored in the memory unit in association with the specific paper feed unit from which the recording paper is fed before the recording paper is fed, from the reference image forming time stored in the memory unit.