Mixture preventing valve includes first flow passage allowing a first fluid to flow; second flow passage allowing a second fluid to flow; communication passage allowing first flow passage and second flow passage to communicate with each other; valve body that opens/closes communication passage; and valve seat provided on an inner wall surface of the communication passage that contacts valve body. Valve body includes first valve body piece and second valve body piece arranged side by side in a state of being separated from each other in a movement direction of valve body. Valve seat contacts respective outer peripheral surfaces of first valve body piece and second valve body piece. In the inner wall surface, a portion where valve seat contacts outer peripheral surface of first valve body piece, and a portion where valve seat contacts outer peripheral surface of the second valve body piece exist on a same plane.