A mixture preventing valve comprising: a first flow passage for allowing a first fluid to flow; a second flow passage for allowing a second fluid to flow; a communication passage for allowing the first flow passage and the second flow passage to communicate with each other; a valve body (17, 30) that moves to open and close the communication passage; and a valve seat that is provided on an inner wall surface of the communication passage, and comes into contact with the valve body (17, 30), wherein the valve body (17, 30) includes a first valve body piece (17a, 31) and a second valve body piece (17b, 32) that are arranged side by side in a state of being separated from each other in a movement direction of the valve body (17, 30), wherein the valve seat comes into contact with respective outer peripheral surfaces of the first valve body piece (17a, 31) and the second valve body piece (17b, 32), and in the inner wall surface, a portion where the valve seat comes into contact with the outer peripheral surface of the first valve body piece (17a, 31), and a portion where the valve seat comes into contact with the outer peripheral surface of the second valve body piece (17b, 32) exist on a tapered surface whose section including a shaft of the valve body (17, 30) is a straight line, or a spherical surface whose section is a curved line, and wherein the second valve body piece (17b, 32) is disposed at a position closer to the second flow passage than the first valve body piece (17a, 31) in the movement direction, and is movable relatively to the first valve body piece (17a, 31) in the movement direction, Z characterized in that: a hollow block part is disposed around the communication passage, an inner space of the hollow block part adjacent to the communication passage in a direction intersecting with the movement direction is provided, and the inner space is opened to atmosphere through an intermediate pipe connected to a lower part of the hollow block part, and when the second valve body piece (17b, 32) relatively moves in such a direction as to approach the first valve body piece (17a, 31) in the movement direction, the inner space communicates with the communication passage and the second flow passage, while the outer peripheral surface of the first valve piece is kept in contact state with the valve seat. a first flow passage for allowing a first fluid to flow; a second flow passage for allowing a second fluid to flow; a communication passage for allowing the first flow passage and the second flow passage to communicate with each other; a valve body (17, 30) that moves to open and close the communication passage; and a valve seat that is provided on an inner wall surface of the communication passage, and comes into contact with the valve body (17, 30), wherein the valve body (17, 30) includes a first valve body piece (17a, 31) and a second valve body piece (17b, 32) that are arranged side by side in a state of being separated from each other in a movement direction of the valve body (17, 30), wherein the valve seat comes into contact with respective outer peripheral surfaces of the first valve body piece (17a, 31) and the second valve body piece (17b, 32), and in the inner wall surface, a portion where the valve seat comes into contact with the outer peripheral surface of the first valve body piece (17a, 31), and a portion where the valve seat comes into contact with the outer peripheral surface of the second valve body piece (17b, 32) exist on a tapered surface whose section including a shaft of the valve body (17, 30) is a straight line, or a spherical surface whose section is a curved line, and wherein the second valve body piece (17b, 32) is disposed at a position closer to the second flow passage than the first valve body piece (17a, 31) in the movement direction, and is movable relatively to the first valve body piece (17a, 31) in the movement direction, Z characterized in that: a hollow block part is disposed around the communication passage, an inner space of the hollow block part adjacent to the communication passage in a direction intersecting with the movement direction is provided, and the inner space is opened to atmosphere through an intermediate pipe connected to a lower part of the hollow block part, and when the second valve body piece (17b, 32) relatively moves in such a direction as to approach the first valve body piece (17a, 31) in the movement direction, the inner space communicates with the communication passage and the second flow passage, while the outer peripheral surface of the first valve piece is kept in contact state with the valve seat.