An alarm device for signalling a reduction of pneumatic pressure of vehicle tires comprises an oscillator 6 having coil secured to a chassis of a vehicle, and a resonator 1 secured to the peripheral portion of a wheel having a tire adjacent to the coil and consisting of a pressure switch 4 for sensing an abnormal change of tire pressure, and a resonant coil and a capacitor 5 tuned to an electromagnetic wave radiated from the oscillation coil. A signal processing device contains means for sensing a 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 tire pressure and for processing a signal from the sensing means, and an alarm for generating a warning signal in accordance with the output of the signal processing device. The latter comprises an input section 21 having a desired number of terminals, a signal processing section 22 having an arithmetic circuit connected to the input section, a resettable latch circuit 26, 31, and an alarm section 23 operated by the output of the latch circuit whereby an alarm signal is not generated when the speed of the vehicle becomes lower than the prescribed threshold value at normal internal pressure.