A fuel-tank-equipped working machine is provided, which is capable of securing a fuel tank to be large in a state where the fuel-tank-equipped working machine is secured to be compact. A generator 10 as a working machine includes a fuel tank 25 supported at an outer case 12. The working machine 10 includes a tank opening 35 that is formed at the outer case 12, a mounting concave portion 38 that is formed on the fuel tank 25, and a cushion portion 37 that is capable of being housed in the mounting concave portion 38. The tank opening 35 is formed so as to be able to expose an upper half part 25a of the fuel tank 25 to an outside 14 of the outer case 12. Further, the cushion portion 37 has a fitting groove 63 capable of being fitted to an opening rim 36 of the tank opening 35.