A boiler tube having a double-tube structure constituted by an innertube and an outertube integrated with each other. The innertube is made of an alloy consisting essentially of, by weight, 0.02 to 0.15% of C, 0.5 to 3.5% of Si, not greater than 2% of Mn, 25 to 40% of Ni, 20.5 to 27% of Cr, 0.5 to 3% of Mo, not greater than 1% of Nb and the balance Fe and having a substantially fully austenite structure. The outertube is made of an alloy consisting essentially of, by weight, 0.02 to 0.2% of C, not greaterthan 3.5% of Si, not greaterthan 2% of Mn, 33 to 45% of Ni, 30 to 40% of Cr and the balance Fe and having a substantially fully austenite structure. The innertube made of the alloy having the composition specified above exhibits a high resistance to steam oxidation without any reduction in high-temperature strength even at elevated steam temperature, while the outer tube made of the alloy having the composition specified above exhibits a superior resistance to corrosion by coal combustion gas.