An antenna unit (101-105, 503) configured to be attached to window glass for a building, comprising: a radiating element; a wave directing member arranged on an outdoor side with respect to the radiating element; and a conductor arranged on an indoor side with respect to the radiating element, wherein a distance between the radiating element and the wave directing member is denoted as a, wherein a wavelength at an operation frequency of the radiating element is denoted as λ g, wherein a relative permittivity of a medium constituted by a dielectric member between the radiating element and the wave directing member is denoted as ε r, and wherein the antenna unit (101-105, 503) is configured in one of the following (i) to (iii): (i) the distance a is (2.11 × ε r - 1.82) mm or more; (ii) a medium is provided between the radiating element and the wave directing member, the medium includes a space, the distance a is 2.1 mm or more, the antenna unit (101-105, 503) is configured to be attached to the window glass so that a matching member is interposed between the window glass and the wave directing member, and where a relative permittivity of the window glass is denoted as ε r 1, a relative permittivity of the matching member is denoted as ε r 2, and a relative permittivity of a medium between the matching member and the radiating element is denoted as ε r 3, ε r 1 is larger than ε r 2, and ε r 2 is larger than ε r 3; and (iii) the distance a is (0.031 × ε r 2 - 0.065 × ε r + 0.040) × λ g or more. a radiating element; a wave directing member arranged on an outdoor side with respect to the radiating element; and a conductor arranged on an indoor side with respect to the radiating element, wherein a distance between the radiating element and the wave directing member is denoted as a, wherein a wavelength at an operation frequency of the radiating element is denoted as λ g, wherein a relative permittivity of a medium constituted by a dielectric member between the radiating element and the wave directing member is denoted as ε r, and wherein the antenna unit (101-105, 503) is configured in one of the following (i) to (iii): (i) the distance a is (2.11 × ε r - 1.82) mm or more; (ii) a medium is provided between the radiating element and the wave directing member, the medium includes a space, the distance a is 2.1 mm or more, the antenna unit (101-105, 503) is configured to be attached to the window glass so that a matching member is interposed between the window glass and the wave directing member, and where a relative permittivity of the window glass is denoted as ε r 1, a relative permittivity of the matching member is denoted as ε r 2, and a relative permittivity of a medium between the matching member and the radiating element is denoted as ε r 3, ε r 1 is larger than ε r 2, and ε r 2 is larger than ε r 3; and (iii) the distance a is (0.031 × ε r 2 - 0.065 × ε r + 0.040) × λ g or more. (i) the distance a is (2.11 × ε r - 1.82) mm or more; (ii) a medium is provided between the radiating element and the wave directing member, the medium includes a space, the distance a is 2.1 mm or more, the antenna unit (101-105, 503) is configured to be attached to the window glass so that a matching member is interposed between the window glass and the wave directing member, and where a relative permittivity of the window glass is denoted as ε r 1, a relative permittivity of the matching member is denoted as ε r 2, and a relative permittivity of a medium between the matching member and the radiating element is denoted as ε r 3, ε r 1 is larger than ε r 2, and ε r 2 is larger than ε r 3; and the medium includes a space, the distance a is 2.1 mm or more, the antenna unit (101-105, 503) is configured to be attached to the window glass so that a matching member is interposed between the window glass and the wave directing member, and where a relative permittivity of the window glass is denoted as ε r 1, a relative permittivity of the matching member is denoted as ε r 2, and a relative permittivity of a medium between the matching member and the radiating element is denoted as ε r 3, ε r 1 is larger than ε r 2, and ε r 2 is larger than ε r 3; and (iii) the distance a is (0.031 × ε r 2 - 0.065 × ε r + 0.040) × λ g or more.