Abstract: Provided is a sensor device being a signal processing device, including: a capacitor; a sensor unit configured to wirelessly transmit, in a case in which electric power of a predetermined amount or more is accumulated in the capacitor, a first signal (SG1) through use of the electric power accumulated in the capacitor; a reception unit configured to receive a radio wave transmitted from a power transmission device being an external device; and a transmission antenna functioning as a second transmission unit, the transmission antenna being configured to wirelessly transmit, in response to reception of the radio wave, a response signal (SG3) being a second signal generated based on the received radio wave without using the electric power accumulated in the capacitor.