To provide an image forming device by which pressure developing irregularity is prevented from occurring by positioning a pressure developing ball without depending on the high accuracy of parts.

A pressing surface being the lower surface of a pressing body 32 pressed by a pressing spring 35 from the upper part of the pressure developing ball 30 housed in a socket 31 attached to a carriage reciprocatingly moved in directions shown by arrows B and C is formed of two plane surfaces extended downward toward an upstream side and a downstream side in the carrying direction (direction shown by an arrow A) of a microcapsule paper 4. Therefore, when the ball 30 housed in the socket 31 is pressed by the pressing body 32 from the upper part, it is deflected or it meanders toward neither of the upstream side or the downstream side in the carrying direction of the paper 4. Thus, the width of superposed pressure developing lines 36 is uniformized.