Disclosed are an addition circuit that makes it possible to add two vector signals in a high-frequency region, a power amplifier circuit using the same, and a transmission device and communication device using the power amplifier circuit. Disclosed is an addition circuit, wherein first through fourth multipliers (11-14) multiply first and second high-frequency input signals (Si1, Si2) and first and second reference signals (Ref1, Ref2) and output first through fourth direct current output signals (Sd1-Sd4), first and second adders (21, 22) add in-phase signals of these to one another and output fifth and sixth direct current output signals (Sd5, Sd6), fifth and sixth multipliers (15, 16) multiply these and the first and second reference signals (Ref1, Ref2) and output first and second high-frequency output signals (So1, So2), which are then combined, whereby a third high-frequency output signal (So3), in which the first and second high-frequency input signals (Si1, Si2) have been vector added, is output from an output terminal (44). The addition circuit can be used in a high-frequency region.