An object is to prevent thermal fatigue, resulting from variations in temperature interfaces, of piping having a fluid-mixing region where fluids of different temperatures are mixed. Piping 1 having a fluid-mixing region M includes a reducer 5 including an end portion 5e disposed substantially on a central axis of a main pipe 2. Low-temperature water flows through the reducer 5 at a lower velocity than high-temperature water flowing outside the end portion 5e. The low-temperature water and the high-temperature water have different temperatures. The low-temperature water and the high-temperature water are mixed in the fluid-mixing region M downstream of the reducer 5. The reducer 5 has a weld line 5f formed to connect the end portion 5e and an elbow portion 5d together. The distance between the weld line 5f and a downstream end 5a is equal to or larger than the inner diameter D of the main pipe 2.