To provide a useful spanner capable of fitting from the side on a rotated body such as a bolt head or a nut having polygonal cross-section, and fastening and releasing the rotated body by being reciprocally turning in the engagement state.

This spanner comprises a handle part 1, a fixing bifurcated part 2 formed continuous with the handle part including first and second fixing arm parts 21, 22, and an opening variable bifurcated part 3 including first and second engaging arm parts 31, 32 formed between the fixing arm parts. One end parts 311, 321 of the engaging arm parts 31, 32 are connected turnably to each other, free end parts 312, 322 face to an opening part S1 between the fixing arms, and an opening is variable around a turnable connection part 30. The inner faces of the engaging arm parts 31, 32 and the fixing arm part 22 have recessed step parts 317a, 318, 33, 325a, 225 and 226 capable of engaging and disengaging with angular parts C of the rotated body W.