A valve to be provided midway of a flow pipe forming a flow path in order to regulate a flow of liquid in the flow path, the valve comprising: a valve element having a cylindrical outer peripheral surface a centered around a rotation axis, which, in use, is orthogonal to a direction of the flow path of the flow pipe, the valve element being formed with a flow-path hole penetrating the valve element orthogonally to the rotation axis; a holding member holding the valve element while keeping close contact with the outer peripheral surface of the valve element, so as to permit the valve element to make rotational movement about the rotation axis in a first direction and in a second direction opposite to the first direction, the holding member having an inlet port to be connected to an upstream section of the flow pipe and an outlet port to be connected to a downstream section of the flow pipe, the inlet port and the outlet port located on respective opposite sides of the holding member across the valve element in a direction orthogonal to the rotation axis; and a rotational restriction section (19a, 19b) which restricts the rotational movement of the valve element about the rotation axis, wherein: the valve element has an open position where the flow-path hole brings the inlet port and the outlet port into communication with each other to open the flow path and a close position where the flow-path hole is deviated from the inlet port and the outlet port in a direction of the rotational movement to make the valve element block the communication between the inlet port and the outlet port to thereby close the flow path; the rotational restriction section restricts the rotational movement of the valve element so as to stop the rotational movement of the valve element in the first direction at the open position, characterized in that in the open position the outer peripheral surface of the valve element overlaps the inlet port in a specific part of the outer peripheral surface, the specific part being a part lying at a rear side of the flow-path hole with respect to the first direction so as to receive pressure of the liquid having passed through the inlet port to allow the pressure to impart a rotational bias in the first direction to the valve element in the open position. a valve element having a cylindrical outer peripheral surface a centered around a rotation axis, which, in use, is orthogonal to a direction of the flow path of the flow pipe, the valve element being formed with a flow-path hole penetrating the valve element orthogonally to the rotation axis; a holding member holding the valve element while keeping close contact with the outer peripheral surface of the valve element, so as to permit the valve element to make rotational movement about the rotation axis in a first direction and in a second direction opposite to the first direction, the holding member having an inlet port to be connected to an upstream section of the flow pipe and an outlet port to be connected to a downstream section of the flow pipe, the inlet port and the outlet port located on respective opposite sides of the holding member across the valve element in a direction orthogonal to the rotation axis; and a rotational restriction section (19a, 19b) which restricts the rotational movement of the valve element about the rotation axis, wherein: the valve element has an open position where the flow-path hole brings the inlet port and the outlet port into communication with each other to open the flow path and a close position where the flow-path hole is deviated from the inlet port and the outlet port in a direction of the rotational movement to make the valve element block the communication between the inlet port and the outlet port to thereby close the flow path; the rotational restriction section restricts the rotational movement of the valve element so as to stop the rotational movement of the valve element in the first direction at the open position, characterized in that in the open position the outer peripheral surface of the valve element overlaps the inlet port in a specific part of the outer peripheral surface, the specific part being a part lying at a rear side of the flow-path hole with respect to the first direction so as to receive pressure of the liquid having passed through the inlet port to allow the pressure to impart a rotational bias in the first direction to the valve element in the open position. the valve element has an open position where the flow-path hole brings the inlet port and the outlet port into communication with each other to open the flow path and a close position where the flow-path hole is deviated from the inlet port and the outlet port in a direction of the rotational movement to make the valve element block the communication between the inlet port and the outlet port to thereby close the flow path; the rotational restriction section restricts the rotational movement of the valve element so as to stop the rotational movement of the valve element in the first direction at the open position, characterized in that in the open position the outer peripheral surface of the valve element overlaps the inlet port in a specific part of the outer peripheral surface, the specific part being a part lying at a rear side of the flow-path hole with respect to the first direction so as to receive pressure of the liquid having passed through the inlet port to allow the pressure to impart a rotational bias in the first direction to the valve element in the open position.