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 from the first predictive residual; extracting a second feature quantity indicating a second similarity between lines of the second predictive residual from the second predictive residual; extracting a third feature quantity indicating a third similarity between lines of the input image in a frame structure of the object block from the input image; extracting a fourth feature quantity indicating a fourth similarity between lines of the input image in a field structure of the object block from the input image; selecting one prediction mode from the frame-based prediction mode and the field-based prediction mode based on the first feature quantity, the second feature quantity, the third feature quantity and the fourth feature quantity; and inter-frame-coding the input motion picture in units of block in accordance with the selected prediction mode.