In a container spout portion included in a container body and configured to be sealed as a seal portion comes into contact with an inner circumferential face in association of attachment thereto of a cap having the seal portion therein, an arcuate portion convex inwards is formed between a top face and the inner circumferential face of the container spout portion. A tangential line to the arcuate portion starts from either a joining point between the arcuate portion and the top face or from a crossing point between a virtual extension line of the arcuate portion and the top face or a virtual extension line of the top face. An angle θ formed between the top face and the tangential line ranges from 30 to 60 degrees.