A transmission device includes a signal source configured to output a local signal with a first frequency, a first amplifier configured to amplify the local signal output from the signal source, a first mixer configured to mix a first input signal with an intermediate frequency and the local signal amplified by the first amplifier and to output a first output signal, and a second mixer configured to mix the first output signal output from the first mixer and the local signal amplified by the first amplifier and to output a second output signal.