A small-sized construction machine comprising: a self-propelled lower traveling structure, an upper revolving structure rotatably mounted on said lower traveling structure, and a working mechanism provided on a front part side of said upper revolving structure, wherein said upper revolving structure includes: a revolving frame forming a support structural body, a prime mover mounted on said revolving frame, a fuel tank having a fuel filling port supplying a fuel to an outer surface and storing the fuel to be supplied to said prime mover, an operator's seat platform provided on said revolving frame by covering an upper side of said fuel tank and on which said operator's seat is mounted, and a cab made of a box body covering a periphery of said operator's seat mounted on said operator's seat platform, wherein said fuel tank is disposed between said revolving frame and said operator's seat platform, brought closer to an outer surface side of said cab, and a fuel filling opening surrounding said fuel filling port of said fuel tank is provided on the outer surface of said cab characterized in that a level gauge displaying a liquid level in said fuel tank is provided on the outer surface of said fuel tank; and a level gauge opening for visually checking said level gauge from outside of said cab is provided, wherein the fuel filling opening and the level gauge opening are capped by a side surface cover provided on the outer surface of said cab and capable of being opened and closed. a self-propelled lower traveling structure, an upper revolving structure rotatably mounted on said lower traveling structure, and a working mechanism provided on a front part side of said upper revolving structure, wherein said upper revolving structure includes: a revolving frame forming a support structural body, a prime mover mounted on said revolving frame, a fuel tank having a fuel filling port supplying a fuel to an outer surface and storing the fuel to be supplied to said prime mover, an operator's seat platform provided on said revolving frame by covering an upper side of said fuel tank and on which said operator's seat is mounted, and a cab made of a box body covering a periphery of said operator's seat mounted on said operator's seat platform, wherein said fuel tank is disposed between said revolving frame and said operator's seat platform, brought closer to an outer surface side of said cab, and a fuel filling opening surrounding said fuel filling port of said fuel tank is provided on the outer surface of said cab characterized in that a level gauge displaying a liquid level in said fuel tank is provided on the outer surface of said fuel tank; and a level gauge opening for visually checking said level gauge from outside of said cab is provided, wherein the fuel filling opening and the level gauge opening are capped by a side surface cover provided on the outer surface of said cab and capable of being opened and closed. a revolving frame forming a support structural body, a prime mover mounted on said revolving frame, a fuel tank having a fuel filling port supplying a fuel to an outer surface and storing the fuel to be supplied to said prime mover, an operator's seat platform provided on said revolving frame by covering an upper side of said fuel tank and on which said operator's seat is mounted, and a cab made of a box body covering a periphery of said operator's seat mounted on said operator's seat platform, wherein said fuel tank is disposed between said revolving frame and said operator's seat platform, brought closer to an outer surface side of said cab, and a fuel filling opening surrounding said fuel filling port of said fuel tank is provided on the outer surface of said cab characterized in that a level gauge displaying a liquid level in said fuel tank is provided on the outer surface of said fuel tank; and a level gauge opening for visually checking said level gauge from outside of said cab is provided, wherein the fuel filling opening and the level gauge opening are capped by a side surface cover provided on the outer surface of said cab and capable of being opened and closed.