A filter for filtering noise generated by a differential signal having a specific wavelength transmitted by a first transmission line and a second transmission line is disclosed. The filter includes a multi-layer substrate, a first microstrip line and a second microstrip line. The first and the second transmission lines and the first and the second microstrip lines are disposed at the multi-layer substrate. In addition, one end of the first microstrip line and one end of the second microstrip line are electrically connected to the first and the second transmission lines, respectively, by passing through the vias, and the other end is in a floating state. The impedances of the first and the second microstrip lines match the impedances of the first and the second transmission lines, respectively. Thus, the first and the second microstrip lines may filter the noise generated by the differential signal having the specific wavelength.