To provide a balloon transformer which has a balanced signal- unbalanced signal conversion function and also has a signal multiplexing or demultiplexing function.

This balloon transformer 20 is provided with two unbalanced transmission lines 15 and 16 and one balanced transmission line 17, that is provided between them. The lines 15 and 16 have respectively line parts L11 and L12 and line parts L15 and L16 which are connected serially. The line 17 has a pair of line parts L13 and L14. The parts L11 and L12 respectively have electric length that corresponds to quarter-wavelength of an application center frequency of an unbalanced signal S2, which is inputted and outputted to/from an unbalanced signal terminal 12. The electric length of the parts L13 and L14 compares the parts L11 and L12 with the parts L15 and L16 and adjusts the longer electric length.