A beam scanning plane antenna formed by stacking a Rotman lens portion, and a beam scanning antenna portion in that order, the beam scanning antenna portion including: a power feeding substrate containing a plurality of antenna groups each constituted of irradiating elements, a power feeding line connected to the irradiating elements and first connecting portions connected electromagnetically to the Rotman lens portion; a first grounding conductor having first slots at a position corresponding to the position of the irradiating element; a second grounding conductor having second slots at a position corresponding to the position of the first connecting portions; a first dielectric provided between the first grounding conductor and the power feeding substrate; and a second dielectric provided between the power feeding substrate and the second grounding conductor, the Rotman lens portion including: a Rotman lens substrate having a Rotman lens pattern, second connecting portions, which are connected to the Rotman lens pattern, adapted to connect the Rotman lens pattern with the first connecting portion electromagnetically, and third connecting portions, which are connected to the Rotman lens pattern for electromagnetically connecting the Rotman lens pattern to a system connecting portion; a third grounding conductor having third slots at a position corresponding to the position of the third connecting portion; a third dielectric provided between the second grounding conductor and the Rotman lens substrate; and a fourth dielectric provided between the Rotman lens substrate and the third grounding conductor, the third and fourth dielectric (31, 32, 33, 34) being constituted by foamed bodies having a low relative dielectric constant; wherein the Rotman lens portion and the beam scanning antenna portion are formed by stacking the third grounding conductor, the fourth dielectric, the Rotman lens substrate, the third dielectric, the second grounding conductor, the second dielectric, the power feeding substrate, the first dielectric and the first grounding conductor in that order. a power feeding substrate containing a plurality of antenna groups each constituted of irradiating elements, a power feeding line connected to the irradiating elements and first connecting portions connected electromagnetically to the Rotman lens portion; a first grounding conductor having first slots at a position corresponding to the position of the irradiating element; a second grounding conductor having second slots at a position corresponding to the position of the first connecting portions; a first dielectric provided between the first grounding conductor and the power feeding substrate; and a second dielectric provided between the power feeding substrate and the second grounding conductor, a Rotman lens substrate having a Rotman lens pattern, second connecting portions, which are connected to the Rotman lens pattern, adapted to connect the Rotman lens pattern with the first connecting portion electromagnetically, and third connecting portions, which are connected to the Rotman lens pattern for electromagnetically connecting the Rotman lens pattern to a system connecting portion; a third grounding conductor having third slots at a position corresponding to the position of the third connecting portion; a third dielectric provided between the second grounding conductor and the Rotman lens substrate; and a fourth dielectric provided between the Rotman lens substrate and the third grounding conductor, the third and fourth dielectric (31, 32, 33, 34) being constituted by foamed bodies having a low relative dielectric constant; wherein the Rotman lens portion and the beam scanning antenna portion are formed by stacking the third grounding conductor, the fourth dielectric, the Rotman lens substrate, the third dielectric, the second grounding conductor, the second dielectric, the power feeding substrate, the first dielectric and the first grounding conductor in that order.