With a dual mode transmitter capable of handling two modulation methods for nonconstant amplitude modulation and constant amplitude modulation, respectively, speed-up of transition between modes is implemented. In a mode handling the constant amplitude modulation, first capacitors included in a low-pass filter constituting an AM loop, and a second capacitor included in an integrator are kept recharged from a first constant-voltage power supply and a second constant-voltage power supply by use of a first switch and a second switch, respectively. By doing so, a value of voltage to be recharged at the time of a mode changeover is decreased, and further, a first variable-gain amplifier starts control of a gain while avoiding a region where the output voltage of the first variable-gain amplifier has slow response against an input voltage.