noun
serious look; straight face
A facial expression that is earnest, unsmiling, or without joking; often used when someone suddenly becomes serious or when a joke is delivered with a deadpan expression.
彼は真顔で冗談を言った。
He told a joke with a straight face.
急に真顔になって、大事な話を始めた。
Suddenly he became serious and started talking about something important.