An optical scanning device comprising: a first mirror that has a first reflecting surface; a second mirror that has a second reflecting surface, and that faces the first mirror; two non-waveguide regions that are disposed between the first and second mirrors (30, 40) and that are spaced apart from each other in a first direction parallel to at least either the first reflecting surface or the second reflecting surface; an optical waveguide region that is disposed between the first and second mirrors (30, 40) and that is sandwiched between the two non-waveguide regions, the optical waveguide region having a higher average refractive index than an average refractive index of each of the non-waveguide regions; and a first adjusting element (60, 78) that changes at least either the average refractive index of the optical waveguide region or a thickness of the optical waveguide region, wherein the optical waveguide region propagates light in a second direction that is parallel to at least either the first reflecting surface or the second reflecting surface and that crosses the first direction, wherein the optical waveguide region and the two non-waveguide regions include respective first regions in which a common material exists, wherein the optical waveguide region or each of the two non-waveguide regions further includes a second region in which a first material having a refractive index (n 1) different from the refractive index (n 2) of the common material exists, wherein the first mirror has a higher light transmittance than a light transmittance of the second mirror and allows part of the light propagating through the optical waveguide region to be emitted through the first mirror in a third direction intersecting a virtual plane parallel to the first and second directions (Y, X), and wherein the first adjusting element (60, 78) changes at least either the average refractive index of the optical waveguide region or the thickness of the optical waveguide region to change the third direction, which is a direction of the light emitted through the first mirror. a first mirror that has a first reflecting surface; a second mirror that has a second reflecting surface, and that faces the first mirror; two non-waveguide regions that are disposed between the first and second mirrors (30, 40) and that are spaced apart from each other in a first direction parallel to at least either the first reflecting surface or the second reflecting surface; an optical waveguide region that is disposed between the first and second mirrors (30, 40) and that is sandwiched between the two non-waveguide regions, the optical waveguide region having a higher average refractive index than an average refractive index of each of the non-waveguide regions; and a first adjusting element (60, 78) that changes at least either the average refractive index of the optical waveguide region or a thickness of the optical waveguide region, wherein the optical waveguide region propagates light in a second direction that is parallel to at least either the first reflecting surface or the second reflecting surface and that crosses the first direction, wherein the optical waveguide region and the two non-waveguide regions include respective first regions in which a common material exists, wherein the optical waveguide region or each of the two non-waveguide regions further includes a second region in which a first material having a refractive index (n 1) different from the refractive index (n 2) of the common material exists, wherein the first mirror has a higher light transmittance than a light transmittance of the second mirror and allows part of the light propagating through the optical waveguide region to be emitted through the first mirror in a third direction intersecting a virtual plane parallel to the first and second directions (Y, X), and wherein the first adjusting element (60, 78) changes at least either the average refractive index of the optical waveguide region or the thickness of the optical waveguide region to change the third direction, which is a direction of the light emitted through the first mirror.