A suspension mechanism has a tube member that is movably attached to an attachment shaft and includes an outer peripheral surface of approximately hemispherical shape, a first attachment shaft retaining member provided on the chassis to swingably retain the attachment shaft with an outer peripheral surface of the tube member abutted on its inside surface, a shaft lower end portion of slip surface shape provided at lowermost part of the attachment shaft, a second attachment shaft retaining member provided on the chassis below the first attachment shaft retaining member and having a vertex equivalent section of approximately conical shape and a slope equivalent section to retain the attachment shaft with the slip surface shape shaft lower end portion abutted onto the vertex equivalent section or the slope equivalent section, and a biasing member that biases the tube member and the shaft lower end portion away from each other.
