The invention relates to an auto tensioner of a chain saw. A spiral guide groove (16) of the disk (15) engages with a protrusion 10a of a slider piece (10) and the slider piece (10) is slidably stored in a guide portion (9) formed in a chain cover (2) via a spring (14). A tensioner (11) comprising a protrusion (11) engaged with a guide bar is slidably stored in the slider piece 10 via a coin spring (12). The protrusion (10a) contacts an outer circumference face of the spiral guide groove on a diameter passing through a rotating center of the disk, and an unshown nut for engaging with a stud bolt 6 provided on an engine cover side is attached on the rotating center of the disk (15). A rotation of the disk enables to fix the chain cover (2) and move the guide bar.