An example system includes a plurality of wireless communication elements and a controller. Respective thresholds for determining whether judgment of decrease in tire air-pressure is to be made or not are changed depending on magnitude of driving torque of the vehicle when the vehicle is performing turning movements. The method also includes obtaining a \u03a8M\u2032 value by measuring the relative rotational angle \u03a8\u2032 of the first rotatable body and obtaining a \u03b8M\u2032 value by measuring the relative rotational angle \u03b8\u2032 of the second rotatable body, using at least an angle sensor having a measurement range of \u03a9. The wire may be continuously wound about the liquid transport element such that each of the portions of the wire defines coils.