This invention can reduce the size of a filter device as compared to a conventional filter device having the same coupling degree. A filter device (1) includes: a substrate (11); strip-shaped conductors (12a1 to 12a5); and a ground conductor layer (13), an area facing an area interposed between adjacent ones of the strip-shaped conductors (between 12a1 and 12a2 and between 12a2 and 12a3) having a first recessed portion (recessed portions 11a1 and 11a2) having a surface covered with the ground conductor layer (13).