1. An image signal processing apparatus, comprising: sequence detecting means for receiving an image signal inputted thereto that is generated by subjecting an image obtained by converting a film image into a video image to double speed conversion, in which signal a film frame begins with a first field and one film frame is formed by four fields, calculating a difference value between pixel signal level of a detected pixel in a present field of the inputted said image signal and pixel signal level of a detected pixel at a position identical with a position of the detected pixel in the present field in a field one frame after said present field of the inputted said image signal, and identifying said first field on the basis of the difference value; motion vector detecting means for detecting a motion vector of the detected pixel in said present field with respect to a field two frames after said present field; and image control means for shifting the position of the detected pixel in said present field in a vector direction of the detected said motion vector within a range of a vector quantity of said motion vector; wherein said image control means sets an amount of shift to a minimum in said first field identified by said sequence detecting means and progressively increases said amount of shift for each field subsequent to said first field.