PURPOSE: To attain low pass filter structure equivalently, to reduce an input/ output transmission loss or reflection and to widen the operating frequency band by adopting open stub structure for the end of a microstrip line.

CONSTITUTION: An end of microstrip lines 30,32 connecting to a connection part 34 is formed to be open stub configuration 30a, 32a. Thus, a capacitor C is formed by the open stubs 30a, 32a and the capacitor C is connected in parallel with an inductance L at the connection part 34 equivalently. Thus, the low pass filter is formed entirely, and the input/output transmission loss and reflection is less at low frequencies and the operating frequency band is widened more than that of a conventional circuit.