A method of controlling an accumulation chamber pressure in an accumulator fuel injection apparatus, which comprises an accumulation chamber for storing a fuel under pressure, a fuel injection nozzle for injecting the fuel in the accumulation chamber into an internal combustion engine, and a fuel pump for pumping the fuel to the accumulation chamber, an accumulator fuel injection apparatus controlling a pump discharge amount of the fuel pump to bring a fuel pressure prevailing in the accumulation chamber to a target pressure, wherein the method comprises the step of: calculating a pump discharge command value of the fuel pump by using feedback, based on a pressure difference between an actual accumulation chamber pressure detected by a fuel pressure sensor and the target pressure of the accumulation chamber; characterized in that the method further comprises the steps of: producing a numerical model of the pump discharge command value of the fuel pump, a disturbance pressure acting on the accumulation chamber, and accumulation chamber pressure using a transfer function of the fuel pump expressing a correlationship of the pressure in the accumulation chamber relative to the effective stroke of the fuel pump, the fuel pump being a plunger pump; estimating a pressure of the disturbance from the numerical model of the pump discharge command value of the fuel pump and the actual accumulation chamber pressure; deriving a compensation value to compensate for the disturbance by using a disturbance observer; and correcting an output calculated by the feedback with the disturbance compensation value derived by the disturbance observer, wherein the method further comprises blocking an output of the disturbance compensation value from the disturbance observer when the derived disturbance compensation value exceeds a given range for a certain period of time in sequence. calculating a pump discharge command value of the fuel pump by using feedback, based on a pressure difference between an actual accumulation chamber pressure detected by a fuel pressure sensor and the target pressure of the accumulation chamber; characterized in that the method further comprises the steps of: producing a numerical model of the pump discharge command value of the fuel pump, a disturbance pressure acting on the accumulation chamber, and accumulation chamber pressure using a transfer function of the fuel pump expressing a correlationship of the pressure in the accumulation chamber relative to the effective stroke of the fuel pump, the fuel pump being a plunger pump; estimating a pressure of the disturbance from the numerical model of the pump discharge command value of the fuel pump and the actual accumulation chamber pressure; deriving a compensation value to compensate for the disturbance by using a disturbance observer; and correcting an output calculated by the feedback with the disturbance compensation value derived by the disturbance observer, wherein the method further comprises blocking an output of the disturbance compensation value from the disturbance observer when the derived disturbance compensation value exceeds a given range for a certain period of time in sequence. producing a numerical model of the pump discharge command value of the fuel pump, a disturbance pressure acting on the accumulation chamber, and accumulation chamber pressure using a transfer function of the fuel pump expressing a correlationship of the pressure in the accumulation chamber relative to the effective stroke of the fuel pump, the fuel pump being a plunger pump; estimating a pressure of the disturbance from the numerical model of the pump discharge command value of the fuel pump and the actual accumulation chamber pressure; deriving a compensation value to compensate for the disturbance by using a disturbance observer; and correcting an output calculated by the feedback with the disturbance compensation value derived by the disturbance observer, wherein the method further comprises blocking an output of the disturbance compensation value from the disturbance observer when the derived disturbance compensation value exceeds a given range for a certain period of time in sequence.