A hot and cold water mixing device, comprising: a hot water supply path which is connected to a hot water supply and which allows hot water having a set temperature set by the hot water supply to flow through; a cold water supply path which is connected to a cold water supply and which allows cold water having an initial temperature to flow through; a hot and cold water adjustment unit configured to mix hot water flowing from the hot water supply path and cold water flowing from the cold water supply path with each other to provide mixed water having an adjusted temperature and an adjusted flow rate; a mixed water path which is connected to the hot and cold water adjustment unit and which allows mixed water to flow through; a water shut-off switch configured to switch between stopping and passing of mixed water, and at least a first temperature sensor and a second temperature sensor which are individually disposed in at least two of the hot water supply path, the cold water supply path, and the mixed water path, characterized in that a mixed water path flowmeter is disposed in the mixed water path and configured to measure a flow rate of mixed water, a flowmeter is disposed only in one of flow paths which has a higher one of a hot water supply assumed flow rate and a cold water supply assumed flow rate determined by the set temperature of the hot water supply and the initial temperature of the cold water supply, the flowmeter being configured to measure a corresponding flow rate of a flow rate of hot water in the hot water supply path or a flow rate of cold water in the cold water supply path, and the hot and cold water adjustment unit is configured to adjust a volume of hot water and a volume of cold water based on measurement values of the flowmeter, the mixed water path flowmeter, and at least the first temperature sensor and the second temperature sensor. a hot water supply path which is connected to a hot water supply and which allows hot water having a set temperature set by the hot water supply to flow through; a cold water supply path which is connected to a cold water supply and which allows cold water having an initial temperature to flow through; a hot and cold water adjustment unit configured to mix hot water flowing from the hot water supply path and cold water flowing from the cold water supply path with each other to provide mixed water having an adjusted temperature and an adjusted flow rate; a mixed water path which is connected to the hot and cold water adjustment unit and which allows mixed water to flow through; a water shut-off switch configured to switch between stopping and passing of mixed water, and at least a first temperature sensor and a second temperature sensor which are individually disposed in at least two of the hot water supply path, the cold water supply path, and the mixed water path, characterized in that a mixed water path flowmeter is disposed in the mixed water path and configured to measure a flow rate of mixed water, a flowmeter is disposed only in one of flow paths which has a higher one of a hot water supply assumed flow rate and a cold water supply assumed flow rate determined by the set temperature of the hot water supply and the initial temperature of the cold water supply, the flowmeter being configured to measure a corresponding flow rate of a flow rate of hot water in the hot water supply path or a flow rate of cold water in the cold water supply path, and the hot and cold water adjustment unit is configured to adjust a volume of hot water and a volume of cold water based on measurement values of the flowmeter, the mixed water path flowmeter, and at least the first temperature sensor and the second temperature sensor.