A clamp configured to close and open a flow path in a tube, comprising: a protrusion section which presses the tube; a locking section having a first engagement section and a second engagement section which form therebetween an engagement groove capable of engagement with the protrusion section; a base section with the locking section erected, an opening/closing section with the protrusion section erected, and a bent section interconnecting the base section and the opening/closing section so that the base section and the opening/closing section are opposed to each other; the first engagement section and the second engagement section being disposed in this order along the direction from a tip of the base section toward the bent section side; and a first engagement release section which releases an engaged state between the first engagement section and the protrusion section; and a second engagement release section which releases an engaged state between the second engagement section and the protrusion section, wherein the flow path in the tube is opened when the engaged states between the protrusion section and the first engagement section and the second engagement section are released, and the second engagement release section is located between the protrusion section and the bent section in a condition where the protrusion section and the locking section are engaged with each other, wherein opening can be performed by elastically deforming and displacing the second engagement section in a direction towards the bent section, and by elastically deforming and displacing the first engagement section in a direction away from the protrusion section, wherein the direction towards the bent section is an opposite direction to the direction away from the protrusion section, characterized in that the second engagement release section projects from the second engagement section toward the bent section side, and a hook piece with which a release pin for putting the second engagement release section into a releasing state can be engaged is provided at a tip of the second engagement release section. a protrusion section which presses the tube; a locking section having a first engagement section and a second engagement section which form therebetween an engagement groove capable of engagement with the protrusion section; a base section with the locking section erected, an opening/closing section with the protrusion section erected, and a bent section interconnecting the base section and the opening/closing section so that the base section and the opening/closing section are opposed to each other; the first engagement section and the second engagement section being disposed in this order along the direction from a tip of the base section toward the bent section side; and a first engagement release section which releases an engaged state between the first engagement section and the protrusion section; and a second engagement release section which releases an engaged state between the second engagement section and the protrusion section, wherein the flow path in the tube is opened when the engaged states between the protrusion section and the first engagement section and the second engagement section are released, and the second engagement release section is located between the protrusion section and the bent section in a condition where the protrusion section and the locking section are engaged with each other, wherein opening can be performed by elastically deforming and displacing the second engagement section in a direction towards the bent section, and by elastically deforming and displacing the first engagement section in a direction away from the protrusion section, wherein the direction towards the bent section is an opposite direction to the direction away from the protrusion section, characterized in that the second engagement release section projects from the second engagement section toward the bent section side, and a hook piece with which a release pin for putting the second engagement release section into a releasing state can be engaged is provided at a tip of the second engagement release section.