Boring tool holder and turning tool

A boring tool holder comprising:

• a base member having a first main surface and a second main surface opposite to the first main surface;
• and
• a cutting insert holding member contiguous to the base member on the first main surface, and disposed in a circumferential direction of the base member as viewed in a direction from the first main surface toward the second main surface,
• a cross-sectional area of the base member in a cross section perpendicular to the direction from the first main surface toward the second main surface and intersecting the base member being larger than a cross-sectional area of the cutting insert holding member in a cross section perpendicular to the direction from the first main surface toward the second main surface and intersecting the cutting insert holding member, wherein
• the base member has a first external circumferential surface that is continuous to each of the first main surface and the second main surface,
• the first external circumferential surface has a curved surface,
• the curved surface of the first external circumferential surface is arcuate when viewed in the direction from the first main surface toward the second main surface, and
• the cutting insert holding member is disposed in a circumferential direction of the curved surface of the first external circumferential surface.