A steel for steel tubes with excellent sulfide stress cracking resistance which comprises, by mass%: C: 0.2 to 0.7%; Si: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: not more than 0.005%; P: not more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: not more than 0.007%; Cr: 0. 1 to 1.5%; Mo: 0.2 to 1.0%; and optionally one or more of Nb: 0.005 to 0.1%, Zr: 0.005 to 0.1%, V: 0.005 to 0.5% and B: 0.0003 to 0.005%; the balance being Fe, Mg and impurities, being characterized in that: the content of Mg in the steel is not less than 1.0 ppm and not more than 5.0 ppm; and non-metallic inclusions of not less than 50% of the total number of those in steel each having the maximum bulk size of not less than 1 μm and comprising two or more elements of Ca, Al, Mg, Ti and Nb and two or more elements of O, S and N have such a morphology that Mg-Al-O-based oxides exist at the central part of the inclusion, Ca-Al-based oxides and/or Ca-Al-based oxysulfides enclose the Mg-Al-O-based oxides, and Ti-containing-carbonitrides or -carbides further exist on a complete or partial periphery of the Ca-Al-based oxides and/or Ca-Al-based oxysulfides, with the proviso that the steel does not have a composition, by mass%, C: 0.27, Si: 0.29, Mn: 0.45, P: 0.006, S: 0.0012, Cr: 0.51, Mo: 0.69, Ti: 0.017, Al: 0.039, N: 0.0044, O: 0.0009, B: 0.0010, V: 0.09, Nb: 0.011, Ca: 0.0004, and Mg: 0.0002, the balance being Fe and impurities; further with the proviso that the steel does not have a composition, by mass%, C: 0.26, Si: 0.28, Mn: 0.46, P: 0.011, S: 0.0005, Cr: 1.03, Mo: 0.68, Ti: 0.013, Al: 0.026, N: 0.0044, O: 0.0010, B: 0.0011, V: 0.09, Nb: 0.013, Ca: 0.0011, and Mg: 0.0003, the balance being Fe and impurities. C: 0.2 to 0.7%; Si: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: not more than 0.005%; P: not more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: not more than 0.007%; Cr: 0. 1 to 1.5%; Mo: 0.2 to 1.0%; and optionally one or more of Nb: 0.005 to 0.1%, Zr: 0.005 to 0.1%, V: 0.005 to 0.5% and B: 0.0003 to 0.005%; the balance being Fe, Mg and impurities, being characterized in that: the content of Mg in the steel is not less than 1.0 ppm and not more than 5.0 ppm; and non-metallic inclusions of not less than 50% of the total number of those in steel each having the maximum bulk size of not less than 1 μm and comprising two or more elements of Ca, Al, Mg, Ti and Nb and two or more elements of O, S and N have such a morphology that Mg-Al-O-based oxides exist at the central part of the inclusion, Ca-Al-based oxides and/or Ca-Al-based oxysulfides enclose the Mg-Al-O-based oxides, and Ti-containing-carbonitrides or -carbides further exist on a complete or partial periphery of the Ca-Al-based oxides and/or Ca-Al-based oxysulfides, with the proviso that the steel does not have a composition, by mass%, C: 0.27, Si: 0.29, Mn: 0.45, P: 0.006, S: 0.0012, Cr: 0.51, Mo: 0.69, Ti: 0.017, Al: 0.039, N: 0.0044, O: 0.0009, B: 0.0010, V: 0.09, Nb: 0.011, Ca: 0.0004, and Mg: 0.0002, the balance being Fe and impurities; further with the proviso that the steel does not have a composition, by mass%, C: 0.26, Si: 0.28, Mn: 0.46, P: 0.011, S: 0.0005, Cr: 1.03, Mo: 0.68, Ti: 0.013, Al: 0.026, N: 0.0044, O: 0.0010, B: 0.0011, V: 0.09, Nb: 0.013, Ca: 0.0011, and Mg: 0.0003, the balance being Fe and impurities. the content of Mg in the steel is not less than 1.0 ppm and not more than 5.0 ppm; and non-metallic inclusions of not less than 50% of the total number of those in steel each having the maximum bulk size of not less than 1 μm and comprising two or more elements of Ca, Al, Mg, Ti and Nb and two or more elements of O, S and N have such a morphology that Mg-Al-O-based oxides exist at the central part of the inclusion, Ca-Al-based oxides and/or Ca-Al-based oxysulfides enclose the Mg-Al-O-based oxides, and Ti-containing-carbonitrides or -carbides further exist on a complete or partial periphery of the Ca-Al-based oxides and/or Ca-Al-based oxysulfides, with the proviso that the steel does not have a composition, by mass%, C: 0.27, Si: 0.29, Mn: 0.45, P: 0.006, S: 0.0012, Cr: 0.51, Mo: 0.69, Ti: 0.017, Al: 0.039, N: 0.0044, O: 0.0009, B: 0.0010, V: 0.09, Nb: 0.011, Ca: 0.0004, and Mg: 0.0002, the balance being Fe and impurities; further with the proviso that the steel does not have a composition, by mass%, C: 0.26, Si: 0.28, Mn: 0.46, P: 0.011, S: 0.0005, Cr: 1.03, Mo: 0.68, Ti: 0.013, Al: 0.026, N: 0.0044, O: 0.0010, B: 0.0011, V: 0.09, Nb: 0.013, Ca: 0.0011, and Mg: 0.0003, the balance being Fe and impurities.