A portable terminal device includes: a receiving unit that receives an action instruction in a predetermined action mode given by a user for one image processing device among a plurality of image processing devices; a radio wave strength detection unit that receives a radio wave for measuring the strength of the radio wave to detect the strength of radio wave of each of the image processing devices; a radio wave strength difference calculation unit that calculates differences in the strengths of radio waves between the image processing device with the maximum strength of radio wave and the other image processing devices; an identification unit that identifies, as the image processing device to which the action instruction received is given, the image processing device having the maximum strength of radio wave; and an instruction unit that gives the action instruction in the action mode to the image processing device.