A THz device includes: an antenna electrode capable of transmitting and receiving a THz wave to free space; first transmission lines capable of transmitting the THz wave, the first transmission lines respectively connected to the antenna electrodes; an active element of which a main electrode is connected to each of the first transmission lines; second transmission lines capable of transmitting the THz wave, the second transmission lines connected to the first active device; pad electrodes respectively connected to the second transmission lines; and a low-pass filter with respect to the THz wave, the low-pass filter connected to the pad electrodes, wherein impedance matching of between the antenna electrode and the active element is performed by an impedance conversion of the first transmission lines. The THz device is capable of the high-efficiency matching between the active element and the antenna due to the impedance conversion effect of the transmission line.