A packing for a reaction tube, wherein a bottom face part of the packing for a reaction tube is used by placing near an inlet side of the reaction tube, the volume is continuously or step-by-step decreased from one end part or midway toward the other end part, the packing further having an opening part to allow passage of fluids, wherein a ratio of the opening part decreases from one end part with a large volume to the other end part with a small volume, wherein when the position of the bottom face part is regarded as 0% and the position of the other end part is regarded as 100% in the height direction of the packing, the opening ratio in the range from not less than 0% to not more than 35% in the height direction is not less than 40% and not more than 70%, the opening ratio in the range from more than 35% to not more than 70% in the height direction is not less than 30% and not more than 40%, and the opening ratio in the range from more than 70% to not more than 100% in the height direction is not less than 10% and not more than 30%, and wherein the bottom face part is an end part having the widest cross-sectional area in the direction at right angle to the height direction of the packing, the volume corresponds to the cross-sectional area in the direction at right angle to the height direction of the packing, the opening ratio is defined as the percentage of the area of openings in the side face part and bottom face part of the packing or a portion thereof to the surface area of the packing, and the midway is a point between the bottom face part and the other end part, the other end part being an end part on the opposite side to the bottom face part. the volume is continuously or step-by-step decreased from one end part or midway toward the other end part, the packing further having an opening part to allow passage of fluids, wherein a ratio of the opening part decreases from one end part with a large volume to the other end part with a small volume, wherein when the position of the bottom face part is regarded as 0% and the position of the other end part is regarded as 100% in the height direction of the packing, the opening ratio in the range from not less than 0% to not more than 35% in the height direction is not less than 40% and not more than 70%, the opening ratio in the range from more than 35% to not more than 70% in the height direction is not less than 30% and not more than 40%, and the opening ratio in the range from more than 70% to not more than 100% in the height direction is not less than 10% and not more than 30%, and wherein the bottom face part is an end part having the widest cross-sectional area in the direction at right angle to the height direction of the packing, the volume corresponds to the cross-sectional area in the direction at right angle to the height direction of the packing, the opening ratio is defined as the percentage of the area of openings in the side face part and bottom face part of the packing or a portion thereof to the surface area of the packing, and the midway is a point between the bottom face part and the other end part, the other end part being an end part on the opposite side to the bottom face part.