An acoustic wave filter device comprising: an unbalanced terminal; first and second balanced terminals (8,9); first and second acoustic wave filters (11,12) that are respectively connected between the unbalanced terminal and the first balanced terminal and between the unbalanced terminal and the second balanced terminal and that have a balanced-unbalanced conversion function; a first parallel resonator connected between the first balanced terminal and a ground potential; and a second parallel resonator connected between the second balanced terminal and the ground potential, characterised in that the acoustic wave filters are longitudinally-coupled-resonator acoustic wave filters, the electrostatic capacitance of the first parallel resonator is different from that of the second parallel resonator, wherein: either the wavelength defined by the electrode finger pitch of an IDT electrode in the parallel resonator having a smaller electrostatic capacitance out of the first and second parallel resonators is shorter than the wavelength defined by the electrode finger pitch of an IDT electrode in the other of the first and second parallel resonators, or the thickness of a piezoelectric thin film defining the wavelength of the parallel resonator having a smaller electrostatic capacitance out of the first and second parallel resonators is different from the thickness of a piezoelectric thin film defining the wavelength of the other of the first and second parallel resonators; and wherein the resonant frequencies of the first and second parallel resonators (13,14) are positioned in the attenuation band at the low frequency side of the filter device passband. an unbalanced terminal; first and second balanced terminals (8,9); first and second acoustic wave filters (11,12) that are respectively connected between the unbalanced terminal and the first balanced terminal and between the unbalanced terminal and the second balanced terminal and that have a balanced-unbalanced conversion function; a first parallel resonator connected between the first balanced terminal and a ground potential; and a second parallel resonator connected between the second balanced terminal and the ground potential, characterised in that the acoustic wave filters are longitudinally-coupled-resonator acoustic wave filters, the electrostatic capacitance of the first parallel resonator is different from that of the second parallel resonator, wherein: either the wavelength defined by the electrode finger pitch of an IDT electrode in the parallel resonator having a smaller electrostatic capacitance out of the first and second parallel resonators is shorter than the wavelength defined by the electrode finger pitch of an IDT electrode in the other of the first and second parallel resonators, or the thickness of a piezoelectric thin film defining the wavelength of the parallel resonator having a smaller electrostatic capacitance out of the first and second parallel resonators is different from the thickness of a piezoelectric thin film defining the wavelength of the other of the first and second parallel resonators; and either the wavelength defined by the electrode finger pitch of an IDT electrode in the parallel resonator having a smaller electrostatic capacitance out of the first and second parallel resonators is shorter than the wavelength defined by the electrode finger pitch of an IDT electrode in the other of the first and second parallel resonators, or the thickness of a piezoelectric thin film defining the wavelength of the parallel resonator having a smaller electrostatic capacitance out of the first and second parallel resonators is different from the thickness of a piezoelectric thin film defining the wavelength of the other of the first and second parallel resonators; and wherein the resonant frequencies of the first and second parallel resonators (13,14) are positioned in the attenuation band at the low frequency side of the filter device passband.