This paper describes real-time gait planning for pushing motion of a humanoid robot. We propose ZMP modification and cycle time modification. ZMP modification is the value which compensates the influence of virtual force to ZMP. Cycle time modification is that the period in double support phase is modified to avoid a robot tipping over. These modifications are calculated from information of reaction force on arms. In these methods, a robot walks while pushing an unknown object. Even if parameters of a object and friction coefficient on the floor vary, the robot keeps on walking while pushing. The effectiveness of the proposed method is confirmed by a simulation and an experiment.