This is a Court cap with no crest which is worn by an emperor at significant ritual ceremonies, with its ei folding over on his head to fix it together with koji with a band of white silk.
天皇が重要な神事において無紋の冠を被り、纓をいったん頭上に上げて折り返し、巾子ごと白い平絹の帯で結んだもの。