A clamp device for a flexible tube comprising: one branch formed with a first projection; another branch formed with a second projection opposing to the first projection; an intermediate part connecting the one branch and the other branch to each other; and an engaging part formed on the other branch adapted to be engaged with the one branch; flow of fluid within the flexible tube laid between the first projection and the second projection being able to be blocked by elastically bending the intermediate part and engaging the engaging part and the one branch with each other to clamp the flexible tube between the first projection and the second projection wherein the clamp device further comprises a positioning part (5a, 8; 11,10a; 5a, 12a, 12b; and 13, 14) for performing a relative positioning of the first and second projections (5 and 6) in the longitudinal direction of the flexible tube under the clamped state of the flexible tube in which the engaging part is engaged with the one branch and wherein said positioning part comprises a fitting part (5a, 11) and a fitted part (8, 10a) each formed on the one branch or the other branch, and wherein the relative positioning of the first projection and the second projection can be achieved by fitting the fitting part (5a, 11) into the fitted part (8, 10a) under a state in which the one branch is engaged by the engaging part of the other branch, characterized in that said engaging part is formed apart from the positioning part (5a, 8; 11, 10a; 5a, 12a, 12b; and 13,14) on the tip end of the other branch and adapted to be engaged by an engaged part formed apart from the positioning part (5a, 8; 11, 10a; 5a, 12a, 12b; and 13, 14) on the tip end of the one branch, and that the first projection, the second projection and the positioning part (5a, 8; 11,10a; 5a, 12a, 12b; and 13, 14) are formed at positions nearer to the intermediate part than the formed positions of the engaged part and the engaging part respectively of the one branch and the other branch. one branch formed with a first projection; another branch formed with a second projection opposing to the first projection; an intermediate part connecting the one branch and the other branch to each other; and an engaging part formed on the other branch adapted to be engaged with the one branch; flow of fluid within the flexible tube laid between the first projection and the second projection being able to be blocked by elastically bending the intermediate part and engaging the engaging part and the one branch with each other to clamp the flexible tube between the first projection and the second projection wherein the clamp device further comprises a positioning part (5a, 8; 11,10a; 5a, 12a, 12b; and 13, 14) for performing a relative positioning of the first and second projections (5 and 6) in the longitudinal direction of the flexible tube under the clamped state of the flexible tube in which the engaging part is engaged with the one branch and wherein said positioning part comprises a fitting part (5a, 11) and a fitted part (8, 10a) each formed on the one branch or the other branch, and wherein the relative positioning of the first projection and the second projection can be achieved by fitting the fitting part (5a, 11) into the fitted part (8, 10a) under a state in which the one branch is engaged by the engaging part of the other branch, characterized in that said engaging part is formed apart from the positioning part (5a, 8; 11, 10a; 5a, 12a, 12b; and 13,14) on the tip end of the other branch and adapted to be engaged by an engaged part formed apart from the positioning part (5a, 8; 11, 10a; 5a, 12a, 12b; and 13, 14) on the tip end of the one branch, and that the first projection, the second projection and the positioning part (5a, 8; 11,10a; 5a, 12a, 12b; and 13, 14) are formed at positions nearer to the intermediate part than the formed positions of the engaged part and the engaging part respectively of the one branch and the other branch.