A heat resistant alloy comprising, in % by weight, over 0.6% to not more than 0.9% of C, up to 2.5% of Si, up to 3.0% of Mn, 20 to 28% of Cr, 8 to 55% of Ni, 0.01 to 0.8% of Ti and 0.05 to 1.5% of Nb, the balance being Fe and inevitable impurities, the value of (Ti+Nb)/C being 0.12 to 0.29 in atomic % ratio. When the alloy further contains up to 0.5% of Zr, the value of (Ti+Nb+Zr)/C is 0.12 to 0.29 in atomic % ratio. When the alloy is heated at a temperature of at least about 800 degrees C., a fine Ti—Nb—Cr carbide or Ti—Nb—Zr—Cr carbide precipitates within grains to thereby retard creep deformation and give an improved creep rupture strength. The alloy is therefore suitable as a material for hydrogen production reforming tubes.