In an insert clamping mechanism of an indexable grooving tool of the present invention, a shaft portion of a clamping tool has a major axis and a minor axis orthogonal to each other on a center axis thereof in a cross-sectional view perpendicular to the center axis. At least one end portion of the both end portions positioned on the major axis in an outer circumference of the shaft portion is formed into a recessed shape or a linear shape and is disposed not to come into contact with an upper wall and a lower wall of an opening-closing operation slit. Support portions are respectively formed in parts adjacent to the end portions on both sides around the center axis along the shaft portion.