1. A motion picture encoding method comprising: searching for a first motion vector between an object block of an input motion picture to be encoded and a reference image for a frame-based prediction mode; searching for a second motion vector between the object block and the reference image for a field-based prediction mode; generating a first predictive residual in accordance with the first motion vector; generating a second predictive residual in accordance with the second motion vector; extracting a first feature quantity indicating a first similarity between lines of the first predictive residual; extracting a second feature quantity indicating a second similarity between lines of the second predictive residual; selecting the frame-based prediction mode in a case where the first similarity is larger than the second similarity based on the first feature quantity and the second feature quantity; selecting the field-based prediction mode in a case where the second similarity is larger than the first similarity; and inter-frame-coding the input motion picture in units of block in accordance with the selected prediction mode.