A waveguide antenna apparatus is provided including a rectangular waveguide having one end short-circuited by a terminating conductor and another end opened. The thermoplastic polymer composition includes 100 parts by mass of a thermoplastic elastomer, 1 to 100 parts by mass of a polyvinyl acetal resin, 5 to 100 parts by mass of a polar group-containing olefin-based copolymer, and 0. The piezoelectric islands have a first surface and a second opposite surface.