A diplexer capable of passing signals of a high frequency band without being attenuated over a wide frequency band, and a high-frequency switch using the diplexer are provided. The diplexer includes a low-pass filter 82 having a parallel resonant circuit formed by a first capacitor C1 and a first inductor L1 and a serial resonant circuit formed by a second capacitor C2 and a second inductor L2. With this structure, two attenuation poles are provided in the low-pass filter 82. Thus, a passband of a high-pass filter 83 can be widened.