A high-frequency component comprising: a diplexer in which a high-pass filter and a low-pass filter are connected in parallel with an antenna port, wherein, among transmission/reception signals of a plurality of communication systems using different frequency bands, a high-frequency transmission/reception signal is made to pass and a low-frequency transmission/reception signal is attenuated by the high-pass filter, and the low-frequency transmission/reception signal is made to pass and the high-frequency transmission/reception signal is attenuated by the low-pass filter; a high-frequency circuit (3, 6, 7; 11, 12, 13) cascaded to the high-pass filter in the diplexer and serving as a front-end for a high-frequency band; and a low-frequency circuit (2, 4, 5; 14, 15, 16) cascaded to the low-pass filter in the diplexer and serving as a front-end for a low-frequency band, wherein the low-pass filter includes an inductor connected between the antenna port and the low-frequency circuit (2, 4, 5; 14, 15, 16) and is constituted by a parallel circuit including said inductor and a first capacitor and a series circuit including the inductor and a second capacitor; transmission characteristics of the low-pass filter are set by the series circuit and the parallel circuit; characterized in that the low-frequency circuit (2, 4, 5; 14, 15, 16) includes a mismatching element (5; 15, 16) which causes an impedance mismatch to occur, and the frequency of an undesired attenuation pole caused by the impedance mismatch, which adversely affects transmission characteristics at the high-pass filter is made to coincide with the frequency of a predetermined attenuation pole in frequency characteristics of the high-pass filter by appropriately setting a frequency of resonance of all the elements of the low-pass filter. a diplexer in which a high-pass filter and a low-pass filter are connected in parallel with an antenna port, wherein, among transmission/reception signals of a plurality of communication systems using different frequency bands, a high-frequency transmission/reception signal is made to pass and a low-frequency transmission/reception signal is attenuated by the high-pass filter, and the low-frequency transmission/reception signal is made to pass and the high-frequency transmission/reception signal is attenuated by the low-pass filter; a high-frequency circuit (3, 6, 7; 11, 12, 13) cascaded to the high-pass filter in the diplexer and serving as a front-end for a high-frequency band; and a low-frequency circuit (2, 4, 5; 14, 15, 16) cascaded to the low-pass filter in the diplexer and serving as a front-end for a low-frequency band, wherein the low-pass filter includes an inductor connected between the antenna port and the low-frequency circuit (2, 4, 5; 14, 15, 16) and is constituted by a parallel circuit including said inductor and a first capacitor and a series circuit including the inductor and a second capacitor; transmission characteristics of the low-pass filter are set by the series circuit and the parallel circuit; characterized in that the low-frequency circuit (2, 4, 5; 14, 15, 16) includes a mismatching element (5; 15, 16) which causes an impedance mismatch to occur, and the frequency of an undesired attenuation pole caused by the impedance mismatch, which adversely affects transmission characteristics at the high-pass filter is made to coincide with the frequency of a predetermined attenuation pole in frequency characteristics of the high-pass filter by appropriately setting a frequency of resonance of all the elements of the low-pass filter.