Abstract: In accordance with a tire air-pressure reduction detecting apparatus of the present invention, if it is judged that the tires all have a normal air pressure, it is judged whether or not a count value (C) is not less than a steady-state count (L.sub.2), and then if the count value (C) is not less than the steady-state count (L.sub.2), a counter (CNT) is decremented. That is, the count value (C) is kept to the steady-state count (L.sub.2) at the lowest, from which incrementation is started. On the other hand, if it is judged that a tire air pressure is reduced, it is judged whether or not the count value(C) is less than an upper counting limit (L.sub.1), and then if the count value (C) is less than the upper counting limit (L.sub.1), the counter (CNT) is incremented. That is, the count value (C) is kept to the upper counting limit (L.sub.1) at the highest, from which decrementation is started. Then, if the count value (C) is not less than a warning threshold count (N.sub.