A transmitter for a tire condition monitoring apparatus includes a pressure sensor for detecting of the internal pressure of a tire, a transmission circuit for generating a transmission signal, and a transmission antenna for transmitting the transmission signal as a radio wave. The transmission antenna is a planar antenna and includes a plate-like transmission conductor for transmitting the radio wave. The transmission antenna is configured such that the radio wave transmitted from the transmission conductor becomes a circularly polarized wave. As a result, the transmitter has a superior transmission performance and a reduced size.