An enlarged image of a thread is displayed and a boundary position between a thread main body and a nap portion at the two ends in the thread thickness direction is inputted by a user on the enlarged image and received. Two boundary marks parallel to the thread length direction are displayed on the enlarged image so that they pass through the boundary positions received and a thread diameter is obtained from the interval between the two boundary marks. A gauge of an appropriate knitter is obtained from the thread diameter obtained by gauge conversion means. A loop shape in the knitted fabric is obtained according to the thread diameter by loop shape conversion means. Furthermore, the thread image is inputted and a knitted fabric sample image is created from the converted loop shape and the thread image by loop simulation so as to serve as a knitted fabric sample.
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