To provide a filter having a simple constitution and a high degree of freedom of design.

The filter comprises first and second line patterns (11, 12) each having a total length substantially equal to 1/2 the wavelength of a pass-band frequency and a resonator (13) located between the first and second line patterns to couple with them so as to function as an open stab which makes connection points between input/output terminals (15, 16) and the first and second line patterns like short-circuited as seen from both ends (14) of the first and the second line patterns.