PURPOSE: To allow the locking and unlocking a lock by providing a key having a superconductive material inside and a temperature controller to keep the key at a fixed temperature under a condition that the key is inserted into a key hole, and changing the condition of the key.

CONSTITUTION: A lock 3 is kept at a fixed temperature by a heater 7 and a thermostat 8. When the key 1 is inserted into the key hole, the temperature of the key 1 is equalized with the lock 3, and further the part in a superconducting condition repels and moves a pin 6 made of permanent magnet by the Meissner effect in an arrangement 2 of superconductors and nonsuperconductors which are embedded in the key 1, thereby permitting an inner cylinder 4 to turn. By changing the temperature of the lock 3 to change the superconductor to a normal conducting condition, the positions of pins 6 are changed to make the locking or unlocking of the lock impossible.