A weft insertion device for a rapier loom that performs weaving by using a tape-shaped flat yarn as a weft, the weft insertion device being a single-rapier weft insertion device with which a rapier head (10, 10'), whose standby position is set on a side opposite to a weft supply side, advances toward the weft supply side, grips the flat yarn on the weft supply side, and inserts the flat yarn as a weft while retreating, the weft insertion device comprising: a yarn guide mechanism that includes a holding device that holds a tip portion of the flat yarn supplied from a weft supply package on the weft supply side and that guides the flat yarn from a position above a movement path of the rapier head (10, 10') to a position below the movement path with respect to an up-down direction, the yarn guide mechanism displacing, independently or in cooperation with the rapier head (10, 10'), the flat yarn in a direction parallel to a width direction of the rapier head (10, 10'), wherein the rapier head (10, 10') includes a head body that includes a yarn inlet portion (11d, 11d'), into which the flat yarn is guided, in a leading end portion thereof, a yarn gripping member, and an operated member that is coupled to the yarn gripping member, wherein, immediately before weft insertion is started, the flat yarn is guided into the yarn inlet portion (11d, 11d') while the flat yarn is displaced by the yarn guide mechanism in the width direction, characterized in that the yarn gripping member is displaceable in the yarn inlet portion (11d, 11d') in a longitudinal direction of the rapier head (10, 10'); the operated member is displaceable in the head body in the longitudinal direction; and the rapier head (10, 10') further includes an operation lever that is operated by engaging with openers (31, 41) that are provided on the weft supply side and the side opposite to the weft supply side and that displaces the operated member in the longitudinal direction, wherein the yarn inlet portion (11d, 11d') is defined by inner side surfaces of the head body that form an angular U-shape in plan view so as to be open toward one side in the width direction and in the up-down direction, and wherein one inner side surface (11b1) of the inner side surfaces that define the yarn inlet portion (11d, 11d'), the one inner side surface (11b1) facing in a retreating direction of the rapier head (10, 10'), functions as one of a pair of gripping surfaces for gripping the flat yarn, and a surface (13b1) of the yarn gripping member that faces the one of the gripping surfaces functions as the other of the gripping surfaces. a yarn guide mechanism that includes a holding device that holds a tip portion of the flat yarn supplied from a weft supply package on the weft supply side and that guides the flat yarn from a position above a movement path of the rapier head (10, 10') to a position below the movement path with respect to an up-down direction, the yarn guide mechanism displacing, independently or in cooperation with the rapier head (10, 10'), the flat yarn in a direction parallel to a width direction of the rapier head (10, 10'), wherein the rapier head (10, 10') includes a head body that includes a yarn inlet portion (11d, 11d'), into which the flat yarn is guided, in a leading end portion thereof, a yarn gripping member, and an operated member that is coupled to the yarn gripping member, wherein, immediately before weft insertion is started, the flat yarn is guided into the yarn inlet portion (11d, 11d') while the flat yarn is displaced by the yarn guide mechanism in the width direction, characterized in that the yarn gripping member is displaceable in the yarn inlet portion (11d, 11d') in a longitudinal direction of the rapier head (10, 10'); the operated member is displaceable in the head body in the longitudinal direction; and the rapier head (10, 10') further includes an operation lever that is operated by engaging with openers (31, 41) that are provided on the weft supply side and the side opposite to the weft supply side and that displaces the operated member in the longitudinal direction, wherein the yarn inlet portion (11d, 11d') is defined by inner side surfaces of the head body that form an angular U-shape in plan view so as to be open toward one side in the width direction and in the up-down direction, and wherein one inner side surface (11b1) of the inner side surfaces that define the yarn inlet portion (11d, 11d'), the one inner side surface (11b1) facing in a retreating direction of the rapier head (10, 10'), functions as one of a pair of gripping surfaces for gripping the flat yarn, and a surface (13b1) of the yarn gripping member that faces the one of the gripping surfaces functions as the other of the gripping surfaces.