PURPOSE: To respond to respective working state by automatically changing a transmission level or transmission signal characteristic when a radio transmitter is used indoors or outdoors in the radio transmitter capable of using selectively a commercial AC power source or a DC power source.

CONSTITUTION: It is detected whether a connected current is the one of the commercial power source or the DC power source by a detecting part 4, and it is judged that the transmitter is used indoors when it is the commercial power source, and that it is used outdoors when it is the DC power source. The level or frequency characteristic of a transmission signal is automatically switched by a transmission circuit control circuit 7 corresponding to detecting output, then, it is introduced to an antenna 8.