To provide a seat stand of a working machine capable of increasing an open space above a floor surface in an operator's cab of the working machine, and optimum for accommodating working tools or the like.

The seat stand 12A of the working machine has an upper surface member 15A for holding a sheet 11 for forming an operator's cab, and a pair of right and left side surface members 15B mounted on a floor plate 13 of the operator's cab. The get-in/get-off side surface member 15B out of the pair of right and left side surface members 15B forms a leg part 18 erected on the floor plate 13 on its rear portion by forming a cutout part 16 in its front portion, and forms an area on the front side of the leg part 18 into an open space 17.