A design system for a knitted fabric including: a base information input unit for a user to input base information, sectionalized to a plurality of knitting units, including a position of a knitting unit, which is a unit for knitting one stitch determined in accordance with the gauge of the flat knitting machine, of a base knitted fabric portion and information associated with a knitting code, representing the knitting operation to be carried out, in the knitting unit; a base memory that stores the base information; a display unit that displays display data based on the base information; and a knitting data preparing portion that prepares knitting data of the knitted fabric based on the base information; the design system characterized by, an inlay information input unit for inputting inlay display data in each knitting unit including an exposed state of an inlay line and a position of the inlay line on one surface side of the base knitted fabric portion wherein the inlay line is arranged so as to thread through the base knitted fabric portion by being sandwiched between stitches allocated to a front needle bed and a back needle bed when knitting the base knitted fabric portion; an inlay information preparing part that prepares inlay information associated with the exposed state and the position, as well as a non-exposed state of the inlay line and a position, of the inlay line based on the inlay display data; an inlay memory that stores the inlay information; a change determining part that references a position α of the inlay line read out from the inlay memory and the knitting code corresponding to the position α read out from the base memory to determine whether or not to change a knitting code of the position α to realize the exposed state or the non-exposed state of the inlay line at the position a; a change code specifying part that specifies a changing knitting code for realizing the exposed state or the non-exposed state of the inlay line at the position α when determined that the knitting code of the position α needs to be changed; and a code change commanding part that commands the knitting data preparing portion to prepare knitting data of the knitted fabric replacing an existing knitting code of the position α with a knitting code specified by the change code specifying part. a base information input unit for a user to input base information, sectionalized to a plurality of knitting units, including a position of a knitting unit, which is a unit for knitting one stitch determined in accordance with the gauge of the flat knitting machine, of a base knitted fabric portion and information associated with a knitting code, representing the knitting operation to be carried out, in the knitting unit; a base memory that stores the base information; a display unit that displays display data based on the base information; and a knitting data preparing portion that prepares knitting data of the knitted fabric based on the base information; the design system characterized by, an inlay information input unit for inputting inlay display data in each knitting unit including an exposed state of an inlay line and a position of the inlay line on one surface side of the base knitted fabric portion wherein the inlay line is arranged so as to thread through the base knitted fabric portion by being sandwiched between stitches allocated to a front needle bed and a back needle bed when knitting the base knitted fabric portion; an inlay information preparing part that prepares inlay information associated with the exposed state and the position, as well as a non-exposed state of the inlay line and a position, of the inlay line based on the inlay display data; an inlay memory that stores the inlay information; a change determining part that references a position α of the inlay line read out from the inlay memory and the knitting code corresponding to the position α read out from the base memory to determine whether or not to change a knitting code of the position α to realize the exposed state or the non-exposed state of the inlay line at the position a; a change code specifying part that specifies a changing knitting code for realizing the exposed state or the non-exposed state of the inlay line at the position α when determined that the knitting code of the position α needs to be changed; and a code change commanding part that commands the knitting data preparing portion to prepare knitting data of the knitted fabric replacing an existing knitting code of the position α with a knitting code specified by the change code specifying part.