A clamping device (1, 1') comprising: a one end portion having a first projection on an inner side and a locked part at a tip; an other end portion having a second projection at a position facing the first projection and a locking part capable of locking the locked part; a middle portion continuous with the one end portion and with the other end portion; and an insertion hole (H1, H2) allowing a flexible tube to be inserted between the first projection and the second projection, wherein a clamped state where the locked part is locked by the locking part is established when the one end portion is pressed and is brought closer to the other end portion, and an unclamped state is established when the locked part locked by the locking part is unlocked by bending the other end portion, the clamping device (1, 1') being configured to clamp the flexible tube in the clamped state where the first projection and the second projection are positioned close to each other and to intercept a flow of a fluid at a clamped position; wherein the other end portion has interfering parts (7, 9), and when the one end portion is further pressed with the locked part being locked by the locking part, the interfering part (7, 9) interfere with an appropriate part and prevent further movement of the locked part, characterised in that, the interfering parts (7, 9) are each a projection integrally provided on the inner side at the tip of the other end portion, and wherein in the clamped state, when the other end portion is bent in a direction (β), which is opposite to the direction to the one end portion, the locked part locked by the locking part is unlocked, whereby the unclamped state can be established. a one end portion having a first projection on an inner side and a locked part at a tip; an other end portion having a second projection at a position facing the first projection and a locking part capable of locking the locked part; a middle portion continuous with the one end portion and with the other end portion; and an insertion hole (H1, H2) allowing a flexible tube to be inserted between the first projection and the second projection, wherein a clamped state where the locked part is locked by the locking part is established when the one end portion is pressed and is brought closer to the other end portion, and an unclamped state is established when the locked part locked by the locking part is unlocked by bending the other end portion, the clamping device (1, 1') being configured to clamp the flexible tube in the clamped state where the first projection and the second projection are positioned close to each other and to intercept a flow of a fluid at a clamped position; wherein the other end portion has interfering parts (7, 9), and when the one end portion is further pressed with the locked part being locked by the locking part, the interfering part (7, 9) interfere with an appropriate part and prevent further movement of the locked part, characterised in that, the interfering parts (7, 9) are each a projection integrally provided on the inner side at the tip of the other end portion, and wherein in the clamped state, when the other end portion is bent in a direction (β), which is opposite to the direction to the one end portion, the locked part locked by the locking part is unlocked, whereby the unclamped state can be established.