An apparatus for alarming a decrease in tire air-pressure which alarms the
decrease in inner pressure of the tire by using a judged value calculated
based on wheel speed information obtained from tires fitted to a
four-wheeled vehicle. The apparatus comprises calculating means for
calculating the judged value based on the wheel speed information on each
of the tires, the calculating means being connected to one of on-vehicle
LANs, via which the wheel speed information on each of the tires is
transmitted. The calculates means captures the wheel speed information on
each of the tires via one of the on-vehicle LANs. The wheel speed
information is converted into the digital data, which is then captured
into the apparatus for alarming the decrease in tire air-pressure via the
on-vehicle LAN, thus remarkably reducing the number of lines laid in the
vehicle in comparison with the prior art.