To solve the problem in which a hose has a large tolerance of an outer diameter, and a hose having a maximum tolerance dimension is excessively pressed when a clamp is adapted to a minimum tolerance dimension, and conversely a hose having the minimum tolerance dimension cannot be tightly locked when a clamp is adapted to the maximum tolerance dimension.

A clamp structure has a pair of clamp pieces formed by curving an elastic plate material in the longitudinal direction, a hinge portion connecting one end portions of the clamp pieces with each other, a locking portion formed in the other end portion of one of the clamp pieces, a lock receiving portion formed in the other end portion of the other clamp piece and engaged with the locking portion, and an engagement portion disposed in an intermediate portion of one of the clamp pieces. When a fitting point is formed by fitting the locking portion to the lock receiving portion, an inner diameter in the direction from the hinge portion to the fitting point is longest in an inner diameter of the opening shape formed by the pair of clamp pieces, and an inner wall of the opening shape has at least two pairs of projections provided at positions opposed to each other with respect to the center of the opening shape.