PURPOSE: To prevent accidents in swimming, by alarming a rescue signal in case of danger.

CONSTITUTION: A knock pin 28 projected from an operating section 1 breaks a waterproof seal 46 of a capsule base 35, and knocks a needle 43 like an injection needle to pierce a sealing plate 41. Gas delivered from an unsealed glass vessel 39 is led through a lateral hole 47 of the needle 43, a conduit 45 and a sound generator 37 to a float 38, and expands the float 38 to open a crevice 48 as shown by a chain line 49. In this manner, the float 38 is separated from the capsule base 35, and at the same time, a buoy 31 gets away from the capsule 34 with the aid of pressure of the float 38 expanded by the gas. The buoy 31 is separated from the operating section, accompanying a thread 32 of the operating section 1, and floats on the surface of water. Under such a circumference, the float 38 is expanded by the injection action of the gas, and simultaneously the sound generator 37 is operated to generate an alarm.