A controller of injection molding machine capable of stabilizing a peak pressure of resin appearing when an injection process is switched to a pressure-holding process, by reducing the variation of the peak pressure. Resin pressure Pr is detected at every predetermined sampling period Δt. There is a time lag between the time at which the resin pressure reaches an injection/pressure-holding switch determining resin pressure P(v−p) and the sampling time t2 at which the resin pressure that has reached the value P(v−P) is detected. Hence, a difference Ve between the detected resin pressure value Pr′ and the switch determining resin pressure P(v−p) is obtained. The transition time period T in which pressure should be changed from a pressure-holding start pressure Ps to a pressure-holding pressure Pp is corrected by T=T0−α×Ve to make the transition time period T shorter when the difference Ve is greater, to thereby hold down the peak pressure Pmax. By this, the variation ΔPmax of the peak pressure Pmax is reduced. Since the pressure is kept uniform, the quality of a molded produce improves.