1. A signal processing unit, comprising: circuitry 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; determine whether the reference region is a region having an edge direction of a shallow angle, wherein in an event the reference region is determined to be not the region having the edge direction of the shallow angle, the circuitry determines whether the reference region is the high-frequency signal region in the specific direction; and in an event 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 a first interpolated value of the phase difference pixel by a gain multiple interpolation technique to a second interpolated value of the phase difference pixel by a peripheral pixel interpolation technique.