A mixture preventing valve includes a first flow passage (11) for allowing a first fluid to flow; a second flow passage (12) for allowing a second fluid to flow; a communication passage (13) for allowing the first flow passage (11) and the second flow passage (12) to communicate with each other; a valve body (17) that moves to open and close the communication passage (13); and a valve seat (18) that is provided on an inner wall surface of the communication passage (13), and comes into contact with the valve body (17). The valve body (17) includes a first valve body piece (17a) and a second valve body piece (17b) that are arranged side by side in a state of being separated from each other in a movement direction of the valve body (17). The valve seat (18) comes into contact with respective outer peripheral surfaces of the first valve body piece (17a) and the second valve body piece (17b). In the inner wall surface, a portion (18a) where the valve seat (18) comes into contact with the outer peripheral surface of the first valve body piece (17a), and a portion (18b) where the valve seat (18) comes into contact with the outer peripheral surface of the second valve body piece (17b) exist on the same plane.