To prevent generation of undue stress at the fixing part of a superconductor to a liquid storage tank by providing a liquid level measuring unit comprising a power supply for feeding a DC current to the superconductor mounted on a support while being insulated electrically, and a voltmeter for measuring the voltage across the superconductor.

Electrodes 13 are secured to the opposite end parts of a support 11 while being mounted on a disc made of a conductive material, e.g. copper, silver or aluminum, and connected with the opposite end parts of a superconductor 12 by soldering. The electrodes 13 are connected through a current lead 14 with a power supply and through a voltage lead 15 with a voltmeter. Since the superconductor 12 is secured in the groove 11a of support 11, centrifugal force acting on the superconductor 12 during rotation of the rotor is transmitted partially to the support 11 and stress being generated in the superconductor 12 is reduced significantly. Consequently, the superconductor can be protected against deterioration of characteristics or breakage.