To provide a plate ridge accessary having a high degree of freedom in response to a large change of the pitch of a roof and to be easily fitted to a corner part formed between a lowering ridge and a horizontal ridge without manufacturing the ridge accessary in field, and to provide a work execution method of a roof.

In the plate ridge accessary for covering a corner part formed between a lowering ridge and a horizontal ridge at a lower end of the lowering ridge, a third accessary structural member is provided so as to cover a clearance generated between a first accessary structural member for covering the lowering ridge and a second accessary structural member for covering the horizontal ridge, and the third accessary structural member connects the first accessary structural member and the second structural member to each other freely to be turned.