In the working machine of the present invention, a tank mounting portion is provided in an upper rotating body, a fuel tank is mounted removably in the tank mounting portion, and engaging portions are formed respectively on both body side of the working machine and the tank side so as to be engaged with each other when the tank is set in the tank mounting portion, thereby preventing in this engaged state the tank from moving in vertical directions and in substantially horizontal directions other than a removable direction of the tank, while a fuel tank dislodgment preventing means is provided to prevent movement in the removable direction of the tank when set in the tank mounting portion. In this working machine, the tank mounting structure can be reduced in both size and weight.