A hybrid working machine, comprising: a hydraulic pump for driving a hydraulic actuator; a generator motor for performing a generator action and a motor action; an engine serving as a common power source for said hydraulic pump and said generator motor; an electric storage device electrically charged by the generator action of said generator motor, the electric storage device being configured such that said generator motor is driven by electric power of the electric storage device so as to perform the motor action; accelerator operating means, a fuel injection amount being changed by an operation of the accelerator operating means so as to control rotation speed of said engine; and generator output controlling means for controlling generator output of said generator motor, wherein said generator output controlling means is configured so as to limit the generator output in a process of accelerating the rotation speed of said engine to set speed determined on the basis of the operation of said accelerator operating means, characterized in that said generator output controlling means is configured so as to release the limit of the generator output at the time of accelerating the rotation speed of said engine at the earlier timing among the following: when the rotation speed of said engine reaches the set speed and after the lapse of a minimum set time preset as the minimum necessary duration for reaching the set speed and stabilizing from the beginning of acceleration of the rotation speed of said engine; and before the rotation speed of said engine reaches the set speed and after the lapse of a maximum set time preset as the sufficient duration for reaching the set speed from the beginning of said acceleration. a hydraulic pump for driving a hydraulic actuator; a generator motor for performing a generator action and a motor action; an engine serving as a common power source for said hydraulic pump and said generator motor; an electric storage device electrically charged by the generator action of said generator motor, the electric storage device being configured such that said generator motor is driven by electric power of the electric storage device so as to perform the motor action; accelerator operating means, a fuel injection amount being changed by an operation of the accelerator operating means so as to control rotation speed of said engine; and generator output controlling means for controlling generator output of said generator motor, wherein said generator output controlling means is configured so as to limit the generator output in a process of accelerating the rotation speed of said engine to set speed determined on the basis of the operation of said accelerator operating means, characterized in that said generator output controlling means is configured so as to release the limit of the generator output at the time of accelerating the rotation speed of said engine at the earlier timing among the following: when the rotation speed of said engine reaches the set speed and after the lapse of a minimum set time preset as the minimum necessary duration for reaching the set speed and stabilizing from the beginning of acceleration of the rotation speed of said engine; and before the rotation speed of said engine reaches the set speed and after the lapse of a maximum set time preset as the sufficient duration for reaching the set speed from the beginning of said acceleration. when the rotation speed of said engine reaches the set speed and after the lapse of a minimum set time preset as the minimum necessary duration for reaching the set speed and stabilizing from the beginning of acceleration of the rotation speed of said engine; and before the rotation speed of said engine reaches the set speed and after the lapse of a maximum set time preset as the sufficient duration for reaching the set speed from the beginning of said acceleration.