Provided is a coaxial waveguide converter and a ridge waveguide that are insusceptible to manufacturing variances over a broad bandwidth. The coaxial waveguide converter includes a ridge waveguide (10) including a ridge (11) and a coaxial line (20). A projection (12) projecting toward a side of a waveguide space (13) is provided in the ridge (11), an amount of projection of the projection (12) decreases gradually from an end surface of the ridge waveguide (10) on a side of the coaxial line along a waveguide direction and an inner conductor (21) of the coaxial line (20) is inserted in the through-hole (14) at a position displaced from a center of the ridge waveguide (10) in a direction perpendicular to a direction in which the projection (12) projects in the end surface of the ridge waveguide (10) on the side of the coaxial line.