A can-manufacturing device performing a diameter-reduction process on a trunk part of a can-body having a closed-end cylindrical shape with an open end, comprising: a can-body pocket which holds the can-body by being in contact with a part of an outer circumference surface of a trunk part of the can-body; a base pad which presses a bottom part of the can-body along a can-axis direction of the can-body by advancing and retreating along the can-axis direction with respect to the can-body pocket; a die which is disposed to face the base pad along the can-axis direction interposing the can-body pocket therebetween so as to perform a diameter-reduction process on the trunk part by press-inserting the trunk part of the can-body along the can-axis direction, characterized by: a clamping ring which protrudes from the base pad toward the die and is provided so as to move back and forth along with the base pad along the can-axis direction with respect to the can-body pocket, wherein the can-body pocket has a plurality of holding surfaces extending in the can-axis direction along the trunk part of the can-bod and being separated circumferentially from each other along the outer circumference surface; and the clamping ring has a plurality of arc-shaped clamping surfaces extending in the can-axis direction along the trunk part of the can-body and being inserted and extracted between the holding surfaces, so that a clamping surface is formed to clamp the outer circumference surface around substantially whole circumference in the vicinity of the bottom part of the can-body held by the can-body pocket by inserting the arc-shaped clamping surface of the clamping ring between the holding surfaces of the can-body pocket. a can-body pocket which holds the can-body by being in contact with a part of an outer circumference surface of a trunk part of the can-body; a base pad which presses a bottom part of the can-body along a can-axis direction of the can-body by advancing and retreating along the can-axis direction with respect to the can-body pocket; a die which is disposed to face the base pad along the can-axis direction interposing the can-body pocket therebetween so as to perform a diameter-reduction process on the trunk part by press-inserting the trunk part of the can-body along the can-axis direction, characterized by: a clamping ring which protrudes from the base pad toward the die and is provided so as to move back and forth along with the base pad along the can-axis direction with respect to the can-body pocket, wherein the can-body pocket has a plurality of holding surfaces extending in the can-axis direction along the trunk part of the can-bod and being separated circumferentially from each other along the outer circumference surface; and the clamping ring has a plurality of arc-shaped clamping surfaces extending in the can-axis direction along the trunk part of the can-body and being inserted and extracted between the holding surfaces, so that a clamping surface is formed to clamp the outer circumference surface around substantially whole circumference in the vicinity of the bottom part of the can-body held by the can-body pocket by inserting the arc-shaped clamping surface of the clamping ring between the holding surfaces of the can-body pocket. a clamping ring which protrudes from the base pad toward the die and is provided so as to move back and forth along with the base pad along the can-axis direction with respect to the can-body pocket, wherein the can-body pocket has a plurality of holding surfaces extending in the can-axis direction along the trunk part of the can-bod and being separated circumferentially from each other along the outer circumference surface; and the clamping ring has a plurality of arc-shaped clamping surfaces extending in the can-axis direction along the trunk part of the can-body and being inserted and extracted between the holding surfaces, so that a clamping surface is formed to clamp the outer circumference surface around substantially whole circumference in the vicinity of the bottom part of the can-body held by the can-body pocket by inserting the arc-shaped clamping surface of the clamping ring between the holding surfaces of the can-body pocket.