To provide a tightening device by which the loosening of a nut is prevented and a member can be tightened surely.

A bolt hole penetrated from a bottom surface 2 to an upper surface is installed screwingly. An upper slit 3 is installed on this upper surface so as to cross the bolt hole. The bottom surface 2 is formed in an inverted V shape and formed so that the height from the bottom surface to the upper surface can be reduced gradually as it approaches from a peripheral edge to the bolt hole. A bottom slit 2b is installed toward the upper surface from the top 2a of the inverted V shape bottom surface. The position of the upper slit 3 is a place in coping with the bottom slit 2b and formed so that the directions of the upper slit 3 and the bottom slit 2b may nearly coincide with each other.