A signal processing apparatus includes one or more processors. The one or more processors perform first-type signal processing on an input signal using a neural network, and output a first-type output signal. The one or more processors convert the first-type output signal into a second-type output signal for calculating a first-type loss related to accuracy of second-type signal processing performed by another signal processing device The one or more processors calculate the first-type loss based on the second-type output signal and a correct signal. The one or more processors optimize parameters of the neural network based on the first-type loss.