A force sensor according to the present invention is configured to detect at least one component among components of a force in each axis direction in an XYZ three-dimensional coordinate system and a moment around each axis, and includes: a support body arranged on an XY plane; a deformation body joined to the support body; and a detection circuit that outputs an electric signal indicating a force applied on the deformation body. The deformation body includes a first deformation portion having a first spring constant and a second deformation portion having a second spring constant different from the first spring constant, and the detection circuit outputs, in response to an applied force, a first electric signal corresponding to the deformation of the first deformation portion and a second electric signal corresponding to the deformation of the second deformation portion and determines whether the force sensor functions normally based on a change of a ratio between the first electric signal and the second electric signal, both corresponding to the applied force.