To provide a semi-coaxial filter of a folding structure, whose size is suppressed from becoming large even in a comparatively low frequency band.

A partition wall 11 is arranged in a conductor case 10 whose cross section is rectangular. Folding space is formed by the partition wall 11. A plurality of resonators 23 are arranged on an inner wall of the conductor case 10. A plurality of resonators 22 and 21 are disposed so that a central axis is positioned on an extension of a central axis of the resonator 23. A coaxial line 40 for mediating connection of the nearby resonators 23 and 22 is installed in the space. Thus, miniaturization is realized while having the folding structure.