A yarn feeding device of a flatbed knitting machine, comprising: a yarn feeding arm (12, 17, 87) that extends downward from a base supported so as to be slidable along a travel rail, the yarn feeding arm (12, 17, 87) being connected to a yarn feeding port (14, 19, 93); a swing arm (2, 62, 89) that is supported so as to be swingable in front and back directions of a needle bed with respect to the yarn feeding arm (12, 17, 87), the front and back directions of the needle bed corresponding to front and back directions of the yarn feeding arm (12, 17, 87), the swing arm (2, 62, 89) having the yarn feeding port (14, 19, 93) at a lower end portion; a shift mechanism (3, 6, 8) that is capable of shifting the yarn feeding port (14, 19, 93) with respect to the yarn feeding arm (12, 17, 87) between a reference position and a front or back position using the swing arm (2, 62, 89), characterised in that the shift mechanism (3, 6) includes a shift member (31, 61) supported so as to be vertically slidable with respect to the yarn feeding arm (12, 17), and shift operation means (37, 63) for shifting the yarn feeding port (14, 19) between the reference position and the front or back position in accordance with a vertical slide of the shift member (31, 61), and wherein the shift operation means includes a support member that is disposed on a lower end portion of the shift member and supports a vertically middle portion of the swing arm so that the vertically middle portion is vertically slidable, the swing arm having an upper end attached to the yarn feeding arm, and a slope portion that is disposed on the yarn feeding arm to guide the support member so that the support member slides in front and back directions, and wherein the yarn feeding port is shifted between the reference position and the front or back position together with the support member while the support member is being guided so as to slide along the slope portion by the vertical slide of the shift member. a yarn feeding arm (12, 17, 87) that extends downward from a base supported so as to be slidable along a travel rail, the yarn feeding arm (12, 17, 87) being connected to a yarn feeding port (14, 19, 93); a swing arm (2, 62, 89) that is supported so as to be swingable in front and back directions of a needle bed with respect to the yarn feeding arm (12, 17, 87), the front and back directions of the needle bed corresponding to front and back directions of the yarn feeding arm (12, 17, 87), the swing arm (2, 62, 89) having the yarn feeding port (14, 19, 93) at a lower end portion; a shift mechanism (3, 6, 8) that is capable of shifting the yarn feeding port (14, 19, 93) with respect to the yarn feeding arm (12, 17, 87) between a reference position and a front or back position using the swing arm (2, 62, 89), characterised in that the shift mechanism (3, 6) includes a shift member (31, 61) supported so as to be vertically slidable with respect to the yarn feeding arm (12, 17), and shift operation means (37, 63) for shifting the yarn feeding port (14, 19) between the reference position and the front or back position in accordance with a vertical slide of the shift member (31, 61), and wherein the shift operation means includes a support member that is disposed on a lower end portion of the shift member and supports a vertically middle portion of the swing arm so that the vertically middle portion is vertically slidable, the swing arm having an upper end attached to the yarn feeding arm, and a slope portion that is disposed on the yarn feeding arm to guide the support member so that the support member slides in front and back directions, and wherein the yarn feeding port is shifted between the reference position and the front or back position together with the support member while the support member is being guided so as to slide along the slope portion by the vertical slide of the shift member. a support member that is disposed on a lower end portion of the shift member and supports a vertically middle portion of the swing arm so that the vertically middle portion is vertically slidable, the swing arm having an upper end attached to the yarn feeding arm, and a slope portion that is disposed on the yarn feeding arm to guide the support member so that the support member slides in front and back directions, and wherein the yarn feeding port is shifted between the reference position and the front or back position together with the support member while the support member is being guided so as to slide along the slope portion by the vertical slide of the shift member.