A bayonet coupling including an annular groove and bayonet lugs on an inner ring-shaped member and an outer ring-shaped member; and insertion openings formed on either the inner ring-shaped member or the outer ring-shaped member which includes the annular groove. The bayonet lugs includes first bayonet lugs associated with the insertion openings to be insertable into the annular groove through the insertion openings, and a second bayonet lug for which no associated insertion opening is formed. The insertion openings, the first bayonet lugs and the second bayonet lug are shaped so that the second bayonet lug is insertable into the annular groove with axes of the outer and inner ring-shaped members being inclined to each other, and so that the first bayonet lugs are insertable into the annular groove through the insertion openings, respectively, after the second bayonet lug is inserted into the annular groove.