Abstract: An alarm device for informing reduction of pneumatic pressure of tires comprises a group of oscillators each consisting of an oscillator circuit, an electric power supply source thereof, a switch for closing an electric power supply circuit when the internal pressure of the tire is lowered to less than a prescribed value and radiation antenna, a group of receiver tuning circuits having a receiver antenna, a sweep generator, a comparison circuit for generating the gate control signal, a gate circuit for sequentially gating the output of a ratio detection circuit, pulse noise elimination circuits consisting of monostable circuits and smoothing filters, comparators for comparing the smoothed DC signal with reference signals which discriminate the operation of an oscillator, and alarm devices for generating alarm information.