A clamp is provided in a blood collecting device that includes a first tube having one end and another end connected to a blood collecting needle and a blood bag, respectively, a second tube branched from a halfway of the first tube through a branched portion, and a block-off member which is provided in the first tube at the blood-bag side beyond the branched portion and which allows the first tube in a blocked state to be opened when torn, in which the clamp is arranged at the branched portion, and is configured to change a state between a first state in which the second tube is opened and a tearing operation of the block-off member is disabled and a second state in which the second tube is closed and a tearing operation to the block-off member is enabled.