A bending method for a board material using a clamp structure for a manipulator used in a bending machine, the clamp structure comprising: a main clamp disposed on a distal end side of the manipulator; and an auxiliary unit adapted to hold the board material and controlled to move forward and backward relative to the manipulator, the auxiliary unit having a suction holding unit adapted to suction-hold the board material, characterized in that the method comprises: preparing a grip changing unit for the board material at a side of a front table disposed before the bending machine for the board material, the main clamp being rotatable relative to the manipulator in a horizontal plane, the suction holding unit being controlled to move forward from a side portion toward a distal end side of an arm of the manipulator and backward relative to the manipulator and the suction holding unit having a front surface suctioning unit disposed on a front surface side of the suction holding unit; clamp-holding the board material with the main clamp of the manipulator and bending an end portion other than at least one side of the board material using the bending machine; rotating the main clamp by 180 degrees relative to the manipulator, causing the grip changing unit to clamp the one side of the board material that has not been bent and unclamping the main clamp; passing the board material that has been clamped with the grip changing unit to the auxiliary unit by suction holding the bent end portion with the front surface suctioning unit and causing the auxiliary unit to hold the board material; and bending the one side of the board material that has not been bent while holding the bent end portion of the board material with the auxiliary unit. a main clamp disposed on a distal end side of the manipulator; and an auxiliary unit adapted to hold the board material and controlled to move forward and backward relative to the manipulator, the auxiliary unit having a suction holding unit adapted to suction-hold the board material, the method comprises: preparing a grip changing unit for the board material at a side of a front table disposed before the bending machine for the board material, the main clamp being rotatable relative to the manipulator in a horizontal plane, the suction holding unit being controlled to move forward from a side portion toward a distal end side of an arm of the manipulator and backward relative to the manipulator and the suction holding unit having a front surface suctioning unit disposed on a front surface side of the suction holding unit; clamp-holding the board material with the main clamp of the manipulator and bending an end portion other than at least one side of the board material using the bending machine; preparing a grip changing unit for the board material at a side of a front table disposed before the bending machine for the board material, the main clamp being rotatable relative to the manipulator in a horizontal plane, the suction holding unit being controlled to move forward from a side portion toward a distal end side of an arm of the manipulator and backward relative to the manipulator and the suction holding unit having a front surface suctioning unit disposed on a front surface side of the suction holding unit; clamp-holding the board material with the main clamp of the manipulator and bending an end portion other than at least one side of the board material using the bending machine; rotating the main clamp by 180 degrees relative to the manipulator, causing the grip changing unit to clamp the one side of the board material that has not been bent and unclamping the main clamp; passing the board material that has been clamped with the grip changing unit to the auxiliary unit by suction holding the bent end portion with the front surface suctioning unit and causing the auxiliary unit to hold the board material; and bending the one side of the board material that has not been bent while holding the bent end portion of the board material with the auxiliary unit. causing the grip changing unit to clamp the one side of the board material that has not been bent and unclamping the main clamp; passing the board material that has been clamped with the grip changing unit to the auxiliary unit by suction holding the bent end portion with the front surface suctioning unit and causing the auxiliary unit to hold the board material; and bending the one side of the board material that has not been bent while holding the bent end portion of the board material with the auxiliary unit.