A compound needle for a flatbed knitting machine, being a combination of: a needle body suitable to be put in a needle groove arranged side by side in a needle bed of a flatbed knitting machine, having a hook at a front end, and being capable of moving back and forth in a longitudinal direction along the needle groove; and a slider, having a tongue to open and close a hook at a front portion, being capable of relatively moving in the longitudinal direction with respect to the needle body, and provided with: an opening and closing body (12, 13) having the tongue at a front portion and a lower joining part (12c, 13c) on a rear portion; and a base body, disposed upward over the opening and closing body (12, 13) in the needle groove, having an upper joining part to be joined to the lower joining part (12c, 13c) of the opening and closing body (12, 13), wherein the compound needle includes: the a concave part (12d, 13d), a convex part and a joining part of the concave part (12d, 13d); characterised by concave part (12d, 13d), provided on one of the lower joining part (12c, 13c) or the upper joining part, opening on the other side, having a side wall extending from both ends of an opening end to the one side, provided with a narrowing part (12e, 13e) in which a width between the side walls is narrower than a width of the opening end in the longitudinal direction; and the convex part, provided on the other of the lower joining part (12c, 13c) or the upper joining part, provided with a widening part in which a width in the longitudinal direction is wider than the narrowing part (12e, 13e) of the concave part (12d, 13d), inserted into the concave part (12d, 13d) from the other side and joined thereto, capable of passing through the narrowing part (12e, 13e) of the concave part (12d, 13d) midway through insertion by an elastic deformation of the concave part (12d, 13d); and in the joining part of the concave part (12d, 13d) to the convex part, a groove is provided at a midpoint of a thickness of either the concave part (12d, 13d) or the convex part, and the other of the concave part (12d, 13d) or the convex part is inserted into the groove. a needle body suitable to be put in a needle groove arranged side by side in a needle bed of a flatbed knitting machine, having a hook at a front end, and being capable of moving back and forth in a longitudinal direction along the needle groove; and a slider, having a tongue to open and close a hook at a front portion, being capable of relatively moving in the longitudinal direction with respect to the needle body, and provided with: an opening and closing body (12, 13) having the tongue at a front portion and a lower joining part (12c, 13c) on a rear portion; and a base body, disposed upward over the opening and closing body (12, 13) in the needle groove, having an upper joining part to be joined to the lower joining part (12c, 13c) of the opening and closing body (12, 13), wherein an opening and closing body (12, 13) having the tongue at a front portion and a lower joining part (12c, 13c) on a rear portion; and a base body, disposed upward over the opening and closing body (12, 13) in the needle groove, having an upper joining part to be joined to the lower joining part (12c, 13c) of the opening and closing body (12, 13), wherein the compound needle includes: the a concave part (12d, 13d), a convex part and a joining part of the concave part (12d, 13d); characterised by concave part (12d, 13d), provided on one of the lower joining part (12c, 13c) or the upper joining part, opening on the other side, having a side wall extending from both ends of an opening end to the one side, provided with a narrowing part (12e, 13e) in which a width between the side walls is narrower than a width of the opening end in the longitudinal direction; and the convex part, provided on the other of the lower joining part (12c, 13c) or the upper joining part, provided with a widening part in which a width in the longitudinal direction is wider than the narrowing part (12e, 13e) of the concave part (12d, 13d), inserted into the concave part (12d, 13d) from the other side and joined thereto, capable of passing through the narrowing part (12e, 13e) of the concave part (12d, 13d) midway through insertion by an elastic deformation of the concave part (12d, 13d); and the a concave part (12d, 13d), a convex part and a joining part of the concave part (12d, 13d); characterised by concave part (12d, 13d), provided on one of the lower joining part (12c, 13c) or the upper joining part, opening on the other side, having a side wall extending from both ends of an opening end to the one side, provided with a narrowing part (12e, 13e) in which a width between the side walls is narrower than a width of the opening end in the longitudinal direction; and the convex part, provided on the other of the lower joining part (12c, 13c) or the upper joining part, provided with a widening part in which a width in the longitudinal direction is wider than the narrowing part (12e, 13e) of the concave part (12d, 13d), inserted into the concave part (12d, 13d) from the other side and joined thereto, capable of passing through the narrowing part (12e, 13e) of the concave part (12d, 13d) midway through insertion by an elastic deformation of the concave part (12d, 13d); and in the joining part of the concave part (12d, 13d) to the convex part, a groove is provided at a midpoint of a thickness of either the concave part (12d, 13d) or the convex part, and the other of the concave part (12d, 13d) or the convex part is inserted into the groove.