In a coaxially-fed slot array antenna including a coaxial line unit configured with a coaxial inner conductor and a coaxial outer conductor that incorporates the coaxial inner conductor; a feeding waveguide for feeding an electromagnetic wave to the coaxial line unit by way of a feeding slot; and a sub-array configured, corresponding to the coaxial line unit, with a plurality of emission slots formed on the external wall surface of the coaxial outer conductor, the space enclosed by the coaxial inner conductor and the coaxial outer conductor is filled with a foamed dielectric having a predetermined foaming rate. As a result, provision is made for a coaxially-fed slot array antenna that can enhance the flexibility in arranging the emission slots, change the width and the gradient of the main beam, and suppress a grating lobe.