1. A signal processing unit, comprising: a high-frequency signal determination section configured to determine whether a reference region as a peripheral region of a phase difference pixel is a high-frequency signal region in a specific direction based on a pixel signal output from an image pickup section having the phase difference pixel disposed therein; and a synthetic ratio determination section configured to, when the reference region is determined to be not the high-frequency signal region in the specific direction, determine, using a pixel difference of the reference region, a synthetic ratio of an interpolated value of the phase difference pixel by a gain multiple interpolation technique to an interpolated value of the phase difference pixel by a peripheral pixel interpolation technique.