A semiconductor circuitry includes a first circuitry having a differential transistor pair and a pair of current sources connected in series to the differential transistor pair, a pair of transmission lines connected to the differential transistor pair at the opposite side to the current sources, and a second circuitry, connected to a node between the differential transistor pair and the current sources, and configured to test operations of at least the differential transistor pair and a latter-stage circuity connected to the transmission lines, in the state where the current outputs of the pair of current sources are stopped.