A power combiner/distributor including first, second, and third waveguides connected with each other in a planar shape, and for either one of distributing power inputted from the first waveguide to the second and third waveguides and combining powers inputted from the second and third waveguides to input the combined power to the first waveguide is provided. The power combiner/distributor includes a branch circuit connected with the first waveguide and for branching a transmission path formed in the first waveguide into first and second transmission paths, and decoupling circuits connected with the branch circuit and also to the second and third waveguides, respectively, the decoupling circuits having a power losing resonator coupled to the second and third waveguides, resonating within an operation frequency band, and causing a power loss.