A gap sensor for detecting gaps between a surface of a conductive first member and a surface of a conductive second member facing the first member by use of a capacitance measuring technique, comprising a probe extending in the direction of a main axis (Xc) and a main body, wherein: the probe has a base end part connected to the main body and a front end side provided with a plurality of electrodes (4a, 4b,...); the probe has a multilayer substrate structure in which a first conductive layer, a second conductive layer, and a third conductive layer are laminated one on another; the first conductive layer defines a first face of the probe and forms an arrayed first electrode group of the plurality of electrodes and a first guard layer electrically disconnected from the first electrode group; the third conductive layer defines a second face of the probe and forms an arrayed second electrode group of the plurality of electrodes at a position corresponding to the first electrode group and a second guard layer electrically disconnected from the second electrode group; the second conductive layer is arranged between the first conductive layer and the third conductive layer and forms a guard electrode (Ge) at a position corresponding to the first and second electrode groups (E1, E2) and signal patterns (Es-1, Es-2) electrically disconnected from the guard electrode, and the signal patterns (Es-1, Es-2) are electrically connected to electrodes of the first and second electrode groups (E1, E2), respectively; and the main body includes a control part, wherein the control part is configured to: apply a probe signal (Vp) to the first guard layer, guard electrode (Ge), and second guard layer; perform a voltage clamp (Vc1, Vc2) of the electrodes of the first and second electrode groups (E1, E2) through the signal patterns (Es-1, Es-2) according to the probe signal (Vp); and detect clamp currents (Is1, Is2) due to the voltage clamp, thereby measuring gaps (d1, d2) at the positions of the electrodes (4a, 4b,...). the probe has a base end part connected to the main body and a front end side provided with a plurality of electrodes (4a, 4b,...); the probe has a multilayer substrate structure in which a first conductive layer, a second conductive layer, and a third conductive layer are laminated one on another; the first conductive layer defines a first face of the probe and forms an arrayed first electrode group of the plurality of electrodes and a first guard layer electrically disconnected from the first electrode group; the third conductive layer defines a second face of the probe and forms an arrayed second electrode group of the plurality of electrodes at a position corresponding to the first electrode group and a second guard layer electrically disconnected from the second electrode group; the second conductive layer is arranged between the first conductive layer and the third conductive layer and forms a guard electrode (Ge) at a position corresponding to the first and second electrode groups (E1, E2) and signal patterns (Es-1, Es-2) electrically disconnected from the guard electrode, and the signal patterns (Es-1, Es-2) are electrically connected to electrodes of the first and second electrode groups (E1, E2), respectively; and the main body includes a control part, wherein the control part is configured to: apply a probe signal (Vp) to the first guard layer, guard electrode (Ge), and second guard layer; perform a voltage clamp (Vc1, Vc2) of the electrodes of the first and second electrode groups (E1, E2) through the signal patterns (Es-1, Es-2) according to the probe signal (Vp); and detect clamp currents (Is1, Is2) due to the voltage clamp, thereby measuring gaps (d1, d2) at the positions of the electrodes (4a, 4b,...). apply a probe signal (Vp) to the first guard layer, guard electrode (Ge), and second guard layer; perform a voltage clamp (Vc1, Vc2) of the electrodes of the first and second electrode groups (E1, E2) through the signal patterns (Es-1, Es-2) according to the probe signal (Vp); and detect clamp currents (Is1, Is2) due to the voltage clamp, thereby measuring gaps (d1, d2) at the positions of the electrodes (4a, 4b,...).