Wireless power transfer (WPT) based microrobot maximize the utilization of microrobot since it can transfer electrical energy wirelessly. Unlikely the static magnetic field microrobot propulsion, the WPT based microrobot uses the magnetic force on magnetic material that generated by time-varying magnetic field. Expecting and evaluating the magnetic force on a magnetic material under wireless power transfer system is important for it determines the moving velocity of microrobot. In this paper, we derive and evaluate a force on a magnetic bar-type material, which is a source of propulsion force of wireless power transfer based microrobot.