It is an object to obtain a transmission line converter in which a rectangular waveguide provided under a transmission line may be reduced in size. In the transmission line converter including: a rectangular waveguide (11); a slot (5) provided in a wall surface of the rectangular waveguide (11); and a transmission line (13) which extends in a direction of a tube axis of the rectangular waveguide (11) and includes signal conductors (2 and 3) and a ground conductor (4), the slot (5) is provided in a narrower wall surface of the rectangular waveguide (11) and has a shape in which a central portion of the slot (5) includes an oblique portion to the tube axis of the rectangular waveguide (11) and at least one of both end portions of the oblique portion includes a portion parallel to the tube axis of the rectangular waveguide (11), and the wall surface of the rectangular waveguide (11) in which the slot (5) is provided is a part of the ground conductor (4).