Slack in the engagement of a shaft (5) and the rotor (3) in a case (1) is prevented by the resiliency of an O ring (6) under radial but not axial compression. Even if heat stress is added, the variable resistor which can secure air-tightness is provided. The rotor (3) and a slider (4) are accommodated in the case (1). A resistance substrate (2) having a collector electrode (25) formed on the central portion of the surface thereof and a circular resistor (24) formed concentrically on the periphery of the collector electrode (25) is engaged with the lower end part of the case (1). A stepped surface (12) supporting the peripheral surface and the bottom surface of the O ring (6) is formed on the upper surface of the case. A boss portion (55) contacting the inner circumference of the O ring (6) and an operating portion (51) projecting on the upper surface of the case (1) and covering the stepped surface (12) is formed on the shaft (5). The inner and outer circumferences of the O ring (6) are pressed and contacted with the stepped surface (12) of the case and the boss portion (55) of the shaft. A gap ( delta ) is provided between either upper or lower end surface of the O ring (6) and the stepped surface (12) or the bottom surface of the operating portion (51). <IMAGE>
