真面目くさる

まじめくさる【真面目くさる】

look sober (as a judge); be as grave as an owl.

●真面目くさった人 a solemn-looking man

・真面目くさって with a solemn [serious, grave] look; demurely

・真面目くさった顔つきをする assume [put on] a serious [solemn] look; wear an expression of religious gravity.

●あの真面目くさった顔を見ると, ついからかいたくなる. When I see such a solemn face, I just can't help wanting to make fun of him.