A knit design system comprising a program creating unit for causing a flat knitting machine to perform knitting based on created modification data that includes information relating to stitches constituting the knitted fabric, and comprising: an input unit configured to input information relating to the actual measurements of the body of a customer; a memory in which the design data that corresponds to a predetermined number of body dimensions set in advance is stored as basic data, wherein the basic data includes the information of the number of stitch rows arranged parallel to each other in the wale direction and a length dimension of each stitch; a selection unit configured to compare an input actual measurement of a body of a customer including at least one of body length and sleeve length with the body dimensions set in the basic data, and select, from the basic data, possible data which is a selected basic data that matches the actual measurement or the closest to the actual measurements, a first calculation unit configured to calculate, based on the actual measurement and a calculation formula that is preset based on the design of the knitted fabric and stored in the memory, a length dimension in the wale direction of the knitted fabric, the length dimension being suitable for the knitted fabric; a second calculation unit configured to calculate, based on a difference between a length dimension set in the possible data and the calculated length dimension, the number of stitch rows along a course direction that are to be inserted or deleted from the possible data to change the length direction of the possible data into the calculated length dimension; a change determination unit configured to determine one or more positions of stitch row in the possible data at which the change in the length direction by addition or deletion of stitch rows, is to be made; and a creating unit configured to create modification data which is the possible data with addition or deletion of stitch rows based on the calculated number of stitch rows and the determined position, wherein, in the basic data, a plurality of variable courses in which at least one of modifications of inserting and deleting a stitch row along the course direction is possible, and a fixed course in which both of the modifications are prohibited are set, and the change determination unit selects, from the variable courses, the stitch rows to be used for the change. an input unit configured to input information relating to the actual measurements of the body of a customer; a memory in which the design data that corresponds to a predetermined number of body dimensions set in advance is stored as basic data, wherein the basic data includes the information of the number of stitch rows arranged parallel to each other in the wale direction and a length dimension of each stitch; a selection unit configured to compare an input actual measurement of a body of a customer including at least one of body length and sleeve length with the body dimensions set in the basic data, and select, from the basic data, possible data which is a selected basic data that matches the actual measurement or the closest to the actual measurements, a first calculation unit configured to calculate, based on the actual measurement and a calculation formula that is preset based on the design of the knitted fabric and stored in the memory, a length dimension in the wale direction of the knitted fabric, the length dimension being suitable for the knitted fabric; a second calculation unit configured to calculate, based on a difference between a length dimension set in the possible data and the calculated length dimension, the number of stitch rows along a course direction that are to be inserted or deleted from the possible data to change the length direction of the possible data into the calculated length dimension; a change determination unit configured to determine one or more positions of stitch row in the possible data at which the change in the length direction by addition or deletion of stitch rows, is to be made; and a creating unit configured to create modification data which is the possible data with addition or deletion of stitch rows based on the calculated number of stitch rows and the determined position, wherein, in the basic data, a plurality of variable courses in which at least one of modifications of inserting and deleting a stitch row along the course direction is possible, and a fixed course in which both of the modifications are prohibited are set, and the change determination unit selects, from the variable courses, the stitch rows to be used for the change.