An outphasing power amplifying device includes a switching signal generating circuit configured to generate a switching pulse signal for switching a class-D power amplifier from two types of sinusoidal wave generated based on amplitude and phase of a modulated wave to be transmitted. The switching signal generating circuit includes: a sin calculation unit and a cos calculation unit for converting phase information of the two types of sinusoidal wave into a quadrature format; a DA converter for converting the quadrature-format phase information; a first filter for removing an aliasing component from the analogue signal; an analogue quadrature modulator for generating a sinusoidal wave from the analogue signals by using a local signal; a second filter for allowing a radio frequency and a component in the vicinity thereof to pass therethrough; and a comparator for converting the sinusoidal wave into a switching pulse signal by comparison with a reference voltage.