PURPOSE: To facilitate engagement by providing a screw thread having a crest spiraling about an annular main body and connecting a pair of front and back flanks in a tooth-like sectional configuration, and designing the start end of the screw thread to be flat from the front flank to the back flank and inclined at an acute back angle relative to the helical axis of the crest.

CONSTITUTION: A screw thread has a crest 14a integrally joining a pair of front and back flanks 14b, 14c in a tooth-like sectional configuration. The crest 14a spirals about the centerline 22a of a main body and along a bolt main body 18 and defines a crest helical axis 24a extending obliquely to the centerline 22a and perpendicularly to the main body. A bolt screw thread 14 has a start end 26a adjacent to a main body front end 18a for first engaging a complementary start end 26b of a nut screw thread 16, and the start end 26a is flat from the front flank 14b to the back flank 14c, is inclined at an acute back angle B1 relative to the helical axis 24a, and is pointed away from the front end 18a. Thus, the threads 14, 16 can be quickly connected together.