An alarm device for informing reduction of pneumatic pressure of vehicle tires by sensing abnormal change of tire internal pressure of a vehicle when running by a pressure sensing switch, by converting the thus sensed abnormal change into an electric signal and by generating an alarm to an operator. This device comprises an oscillator having an oscillation coil fixed on a chassis side of the vehicle; a resonator consisting of a resonance coil and a capacitor fixed to a peripheral portion of the rotating wheel having tire adjacent the oscillation coil and for resonating with an electromagnetic wave radiated from the oscillation coil; a signal processing device containing a sensing means for sensing change in an oscillation condition generated in the oscillator due to a resonant condition of the resonator switched on and off in accordance with abnormal internal pressure of a tire and for processing a signal sensed by this sensing means; and an alarm for generating a warning by the output of this signal processing device.