A recording medium transfer apparatus, comprising: an endless belt wherein a recording medium is configured to be placed on a surface thereof; a plurality of transfer rollers (23,24,25) which are arranged to support the endless belt so that a part of the endless belt is to be flat and configured to intermittently rotate the endless belt; a slide unit which is configured to freely move back and forth between an initial position of a path which is the part of the endless belt that is flat and an end position that is at a downstream side in a rotational direction of the endless belt than the initial position; a driving device which is configured to return the slide unit to the initial position from the end position; a linear encoder which is configured to detect a transfer amount of the slide unit; a calculation unit which is configured to calculate a transfer amount of the recoding medium from a detection result obtained by the linear encoder; and a control unit which is configured to control a transfer amount of the recording medium to be transferred by the transfer rollers (23,24,25) according to the calculated transfer amount, wherein the slide unit is configured to engage with the part of the endless belt that is flat at the initial position before rotational movement of the endless belt starts, to move following the rotational movement of the endless belt, to release an engagement with the endless belt when reaching the end position and to return to the initial position by the driving device when the endless belt is at a stop after the engagement is released, and the calculation unit is configured to calculate the transfer amount of the recording medium according to the detection result obtained by the linear encoder during a movement of the slide unit reaching the end position from the initial position. an endless belt wherein a recording medium is configured to be placed on a surface thereof; a plurality of transfer rollers (23,24,25) which are arranged to support the endless belt so that a part of the endless belt is to be flat and configured to intermittently rotate the endless belt; a slide unit which is configured to freely move back and forth between an initial position of a path which is the part of the endless belt that is flat and an end position that is at a downstream side in a rotational direction of the endless belt than the initial position; a driving device which is configured to return the slide unit to the initial position from the end position; a linear encoder which is configured to detect a transfer amount of the slide unit; a calculation unit which is configured to calculate a transfer amount of the recoding medium from a detection result obtained by the linear encoder; and a control unit which is configured to control a transfer amount of the recording medium to be transferred by the transfer rollers (23,24,25) according to the calculated transfer amount, wherein the slide unit is configured to engage with the part of the endless belt that is flat at the initial position before rotational movement of the endless belt starts, to move following the rotational movement of the endless belt, to release an engagement with the endless belt when reaching the end position and to return to the initial position by the driving device when the endless belt is at a stop after the engagement is released, and the calculation unit is configured to calculate the transfer amount of the recording medium according to the detection result obtained by the linear encoder during a movement of the slide unit reaching the end position from the initial position.