A singular ponderomotive force produced by spatially nonuniform non-monochromatic RF field whose envelope of the amplitude varies at a cyclotron frequency is investigated by a simple analysis of a single charged-particle motion in a static magnetic field. Numerical calculation of the particle orbit shows that a charged-particle is continuously accelerated by the oscillating ponderomotive force synchronizing with the natural cyclotron motion of particle. The singular behaviour of the oscillating ponderomotive force may be applicable to the experiment of ICRF plasma heating.