A dielectric filter, comprising: a multilayer dielectric substrate, which includes a plurality of conductor layers formed so as to be separated apart from each other in a laminating direction, and is configured to propagate a high-frequency signal; a first strip line and a second strip line, which are formed so as to extend in a planar direction in conductor layers that are separated away from each other in the laminating direction; a dielectric waveguide formed of the conductor layers extending in the planar direction and conductor posts (3018,3024,3057) extending in the laminating direction, the dielectric waveguide being formed between the first strip line and the second strip line in the laminating direction of the multilayer dielectric substrate; a first strip line-waveguide converter, which is formed on an upper side of the first strip line in the laminating direction, and is configured to perform transmission line conversion between the dielectric waveguide and the first strip line; and a second strip line-waveguide converter, which is formed on a lower side of the second strip line in the laminating direction, and is configured to perform transmission line conversion between the dielectric waveguide and the second strip line, wherein the first strip line-waveguide converter includes: a first probe having one end connected to the first strip line, and another end arranged so as to oppose the dielectric waveguide; and a first back-short waveguide (2001-2003, 3018) having one end that is short-circuited, and another end connected to the dielectric waveguide so as to oppose the dielectric waveguide, and wherein the second strip line-waveguide converter includes: a second probe having one end connected to the second strip line, and another end arranged so as to oppose the dielectric waveguide; and a second back-short waveguide (2006-2008, 3018) having one end that is short-circuited, and another end connected to the dielectric waveguide so as to oppose the dielectric waveguide, wherein the back-short waveguides include a conductor layer having an aperture whose diameter is smaller than an aperture diameter of a conductor layer in the dielectric waveguide. a multilayer dielectric substrate, which includes a plurality of conductor layers formed so as to be separated apart from each other in a laminating direction, and is configured to propagate a high-frequency signal; a first strip line and a second strip line, which are formed so as to extend in a planar direction in conductor layers that are separated away from each other in the laminating direction; a dielectric waveguide formed of the conductor layers extending in the planar direction and conductor posts (3018,3024,3057) extending in the laminating direction, the dielectric waveguide being formed between the first strip line and the second strip line in the laminating direction of the multilayer dielectric substrate; a first strip line-waveguide converter, which is formed on an upper side of the first strip line in the laminating direction, and is configured to perform transmission line conversion between the dielectric waveguide and the first strip line; and a second strip line-waveguide converter, which is formed on a lower side of the second strip line in the laminating direction, and is configured to perform transmission line conversion between the dielectric waveguide and the second strip line, wherein the first strip line-waveguide converter includes: a first probe having one end connected to the first strip line, and another end arranged so as to oppose the dielectric waveguide; and a first back-short waveguide (2001-2003, 3018) having one end that is short-circuited, and another end connected to the dielectric waveguide so as to oppose the dielectric waveguide, and a first probe having one end connected to the first strip line, and another end arranged so as to oppose the dielectric waveguide; and a first back-short waveguide (2001-2003, 3018) having one end that is short-circuited, and another end connected to the dielectric waveguide so as to oppose the dielectric waveguide, and wherein the second strip line-waveguide converter includes: a second probe having one end connected to the second strip line, and another end arranged so as to oppose the dielectric waveguide; and a second back-short waveguide (2006-2008, 3018) having one end that is short-circuited, and another end connected to the dielectric waveguide so as to oppose the dielectric waveguide, a second probe having one end connected to the second strip line, and another end arranged so as to oppose the dielectric waveguide; and a second back-short waveguide (2006-2008, 3018) having one end that is short-circuited, and another end connected to the dielectric waveguide so as to oppose the dielectric waveguide, wherein the back-short waveguides include a conductor layer having an aperture whose diameter is smaller than an aperture diameter of a conductor layer in the dielectric waveguide.