A measuring jig is attached to an antenna support member of a revolving unit, and used to measure a slewing position of the revolving unit together with an external measurement apparatus. The measuring jig includes a prism mirror and an attachment member. The prism mirror reflects projection light from the external measurement apparatus. The attachment member attaches the prism mirror to the antenna support member. The attachment member is configured to be attached to the antenna support member by sandwiching an attached portion.