A lathe turning tool comprising: a tool main body; and a cutting insert attached to an insert attachment portion at an end of said tool main body, said lathe turning tool processing a workpiece with a cutting edge of said cutting insert while rotating said tool main body and said cutting insert about a center line of the tool as an axis, said cutting edge extending continuously in a tool rotating direction, said cutting insert including a first portion related to cutting and a second portion used for positioning, said first portion having a rake face disposed at said end of the tool, a flank face disposed on an outer circumference of the tool, a cutting edge having a circular shape or an approximately circular shape as seen from an axial end face, and a face facing the insert attachment portion at the end of said tool main body, and said second portion having side surfaces, each side surface including at least one flat plane that is parallel to said center line of the tool or inclined at a prescribed angle to said center line of the tool, said second portion extending continuously to a region surrounded by the face of said first portion that faces the insert attachment portion at the end of said tool main body, and said second portion being fitted into the tool main body to position the cutting insert and to prevent the cutting insert from rotating, wherein a concave portion into which said second portion of said cutting insert is fitted is provided by providing an opening at said end of said tool main body, said concave portion has a flat restraining surface provided to correspond to the flat plane of each side surface of said second portion, characterized in that said flat restraining surface has a width narrower than a width of the flat plane of the side surface of said second portion, and the flat plane of the side surface of the second portion comes into contact with one end of said flat restraining surface when said second portion rotates within said concave portion. a tool main body; and a cutting insert attached to an insert attachment portion at an end of said tool main body, said lathe turning tool processing a workpiece with a cutting edge of said cutting insert while rotating said tool main body and said cutting insert about a center line of the tool as an axis, said cutting edge extending continuously in a tool rotating direction, said cutting insert including a first portion related to cutting and a second portion used for positioning, said first portion having a rake face disposed at said end of the tool, a flank face disposed on an outer circumference of the tool, a cutting edge having a circular shape or an approximately circular shape as seen from an axial end face, and a face facing the insert attachment portion at the end of said tool main body, and said second portion having side surfaces, each side surface including at least one flat plane that is parallel to said center line of the tool or inclined at a prescribed angle to said center line of the tool, said second portion extending continuously to a region surrounded by the face of said first portion that faces the insert attachment portion at the end of said tool main body, and said second portion being fitted into the tool main body to position the cutting insert and to prevent the cutting insert from rotating, wherein a concave portion into which said second portion of said cutting insert is fitted is provided by providing an opening at said end of said tool main body, said concave portion has a flat restraining surface provided to correspond to the flat plane of each side surface of said second portion, characterized in that said flat restraining surface has a width narrower than a width of the flat plane of the side surface of said second portion, and the flat plane of the side surface of the second portion comes into contact with one end of said flat restraining surface when said second portion rotates within said concave portion.