PURPOSE: To prevent malfunction of a pulse generating element and to shorten the time for restart by permitting a bimetal switch to perform its deactuating action quickly after the light emitting tube has lightened up and also to perform its restitution to ON promptly after the tube is put off.

CONSTITUTION: When the light emitting tube 2 lights up at the time of starting, a bimetal piece 21 lies facing the heat emission H in the radial direction with a certain angle as illustrated in Fig. A, so that the heat from the light emitting tube 2 is easily received to initiate bending immediately. Accordingly, the bimetal switch 5 is deactuated quickly after the start. When the light emitting tube 2 lights up, on the other hand, the bimetal piece 21 is in parallel approx. with the radiating heat line H as illustrated in Fig. B. This attitude provides easy reception of the heat from the tube 2, whereby turning-off of the light emitting tube 2 will allow quick restitution of the bimetal piece 21, so that return to ON of the switch 5 can be made extremely fast.