An automatic car tire pressure detecting apparatus constitutes an emission means located at the wheel rim of the tire and a receiving means located at the brake pad. The emission means includes a generator, an emission end and a pressure sensor. The generator has a coil which has a positive and a negative end linked in series with a rectify switch and the emission end. The receiving means includes a magnet located at a corresponding tangent spot of the coil, a receiving end and a microprocessor attached to a display device. When the tire rotates, the magnet continuously cuts the coil to generate electric current to actuate the emission end. The rectify switch determines if the current flowing therethrough is positive. The positive current will be used for actuating the emission end to emit signals which are received by the receiving end thereby the detecting apparatus is able to provide self-generating power supply to transmit signals without an external power supply.