To allow contactless signal transmission by means of magnetic coupling by detecting a size of load on both ends of a solenoid by means of voltage, a current and a phase change of a static current wire while having no mechanical contact.

Tape switches 6 are connected to both ends of a solenoid 3, a ring-shaped solenoid 2 containing a movable iron core and a static current wire 1 passing through inside a closed magnetic path of the ring-shaped solenoid 2 containing the movable iron core are set up inside a guide rail 8. Then, the ring-shaped solenoid 2 containing the movable iron core is made allowable to go up and down in company with an up-and-down operation of baseboard 5 while maintaining a state with a passing-through static current wire. Accordingly, detection of the presence or absence of an obstruction can be made by a control part by means of changes in the voltage, current and phase of the static current wire 1 to be generated during the up-and-down operation of the baseboard 5. Therefore, a low-priced and highly reliable contactless signal transmission device can be obtained.