A waveguide-microstrip line converter, comprising: a waveguide; a dielectric substrate that is connected to cover one end of the waveguide; a strip conductor that is disposed on an end of one surface of the dielectric substrate; a conductor plate that is disposed substantially in a center of the one surface of the dielectric substrate, and connected to the strip conductor; a ground conductor that is disposed on another surface of the dielectric substrate except for a connection region of the waveguide and the dielectric substrate; and a plurality of connection conductors that connect a periphery of the conductor plate and the ground conductor except for a portion that connects the strip conductor and the conductor plate, wherein the ground conductor has an opening (108, 408) formed therein in the connection region of the waveguide and the dielectric substrate, wherein the conductor plate is disposed to cover the opening (108, 408) through intermediation of the dielectric substrate, and wherein the strip conductor and the ground conductor form a microstrip line, characterized in that the plurality of connection conductors is arranged in two lines, each with a segment in the vicinity of the connection portion and a segment in the vicinity of the opening in a longitudinal direction of the microstrip line, wherein said two lines are disposed on opposing sides of the conductor plate so that a distance between the two lines in the segment in the vicinity of the connection portion of the strip conductor and the conductor plate is narrower than a distance between the two lines in the segment in the vicinity of the opening. a waveguide; a dielectric substrate that is connected to cover one end of the waveguide; a strip conductor that is disposed on an end of one surface of the dielectric substrate; a conductor plate that is disposed substantially in a center of the one surface of the dielectric substrate, and connected to the strip conductor; a ground conductor that is disposed on another surface of the dielectric substrate except for a connection region of the waveguide and the dielectric substrate; and a plurality of connection conductors that connect a periphery of the conductor plate and the ground conductor except for a portion that connects the strip conductor and the conductor plate, wherein the ground conductor has an opening (108, 408) formed therein in the connection region of the waveguide and the dielectric substrate, wherein the conductor plate is disposed to cover the opening (108, 408) through intermediation of the dielectric substrate, and wherein the strip conductor and the ground conductor form a microstrip line, characterized in that the plurality of connection conductors is arranged in two lines, each with a segment in the vicinity of the connection portion and a segment in the vicinity of the opening in a longitudinal direction of the microstrip line, wherein said two lines are disposed on opposing sides of the conductor plate so that a distance between the two lines in the segment in the vicinity of the connection portion of the strip conductor and the conductor plate is narrower than a distance between the two lines in the segment in the vicinity of the opening.