Abstract: A hot and cold water mixing device comprises an outercasing having openings 51, 52, 170a and 180a which communicate with a hot water supply passage, a cold water supply passage and a mixed water delivery passage, respectively, and a hollow inner casing 60 formed with two primary water holes communicating with the hot water supply passage and the cold water supply passage, respectively, and with a secondary water hole communicating with the mixed water delivery passage, said primary and secondary water holes being penetrated from the outer surface of the inner casing. The inner casing 60 is inserted into the outer casing 50 with a gap remaining radially therebetween, and further, separating members 77 are arranged within the gap, so that fluid passages 55, 176 and 184 are defined between the outer casing 50 and the inner casing 60. All or a portion of the outer casing 50 is formed using a resin material such as engineering plastics or the like.