A continuous casting method of molten metal in which a hollow cylindrical, conical or truncated cone type refractory-made structure having one or more side holes in a sidewall thereof is disposed in a tundish above a submerged entry nozzle with an central axis of the refractory-made structure aligned vertically to supply molten metal from the tundish to the submerged entry nozzle, wherein: a central axis of the side hole crosses a virtual line extending radially from the center of a horizontal circular cross-section of the refractory-made structure at an intersection thereof with an inner surface of the refractory-made structure, the central axis of side hole being horizontally inclined at an angle θ1 relative to the virtual line at the intersection; the molten metal in the tundish passes from inlet-side openings of side holes which are opened on an outer surface of the refractory-made structure to outlet-side openings thereof which are opened on the inner surface of the refractory-made structure, whereby a swirling flow is generated in the molten metal supplied from the tundish to the submerged entry nozzle while giving a circumferential velocity thereto; and a mean inner diameter 2R of the horizontal circular cross-section of the structure in the region having openings of side holes is 250 to 1,200 mm, the height of section of side hole is 30 to 500 mm, and the angle θ1 is 15 to 80°; and characterized in that: an index P expressed by the following equation satisfies 0.015 m 2 /s≤P≤0.100 m 2 /s, the index P being represented by a flow rate Q of the molten metal, total opening areas S of the side holes, a mean inner radius R of horizontal circular cross-section in a region having openings of side holes, and the angle θ1. P = R × Q / S × Sinθ 1 a central axis of the side hole crosses a virtual line extending radially from the center of a horizontal circular cross-section of the refractory-made structure at an intersection thereof with an inner surface of the refractory-made structure, the central axis of side hole being horizontally inclined at an angle θ1 relative to the virtual line at the intersection; the molten metal in the tundish passes from inlet-side openings of side holes which are opened on an outer surface of the refractory-made structure to outlet-side openings thereof which are opened on the inner surface of the refractory-made structure, whereby a swirling flow is generated in the molten metal supplied from the tundish to the submerged entry nozzle while giving a circumferential velocity thereto; and a mean inner diameter 2R of the horizontal circular cross-section of the structure in the region having openings of side holes is 250 to 1,200 mm, the height of section of side hole is 30 to 500 mm, and the angle θ1 is 15 to 80°; and characterized in that: an index P expressed by the following equation satisfies 0.015 m 2 /s≤P≤0.100 m 2 /s, the index P being represented by a flow rate Q of the molten metal, total opening areas S of the side holes, a mean inner radius R of horizontal circular cross-section in a region having openings of side holes, and the angle θ1. P = R × Q / S × Sinθ 1 an index P expressed by the following equation satisfies 0.015 m 2 /s≤P≤0.100 m 2 /s, the index P being represented by a flow rate Q of the molten metal, total opening areas S of the side holes, a mean inner radius R of horizontal circular cross-section in a region having openings of side holes, and the angle θ1. P = R × Q / S × Sinθ 1