A tire inflation pressure sensing apparatus includes a transmitter on a
wheel of a vehicle and a receiver on a body of the vehicle. The
transmitter includes a pressure sensor working to generate a pressure
signal and a charging unit configured to be charged with electrical power
via a radio wave from the receiver. The receiver performs a "packet
charge" for the charging unit of the transmitter, during which operation
of the receiver is changed alternately between an ON-mode and an
OFF-mode. When the receiver receives in the OFF-mode an informing signal
from the transmitter, it stops the packet charge to receive the pressure
signal sent out from the transmitter following the informing signal. The
transmitter starts, when the electrical power charged in the charging
unit has increased to a predetermined level, to repeatedly send out the
informing signal a plurality of times, and then sends out the pressure
signal.