A seamless steel tube for an airbag accumulator which can be manufactured by heat treatment of normalizing without quenching and tempering and which has a tensile strength of at least 850 MPa and resistance to bursting at -20° C has a stee; composition comprising, in mass %, C: 0.08 - 0.20%, Si: 0.1 - 1.0%, Mn: 0.6 - 2.0%, P: at most 0.025%, S: at most 0.010%, Cr: 0.05 - 1.0%, Mo: 0.05 - 1.0%, Al: 0.002 - 0.10%, at least one of Ca: 0.0003 - 0.01%, Mg: 0.0003 - 0.01%, and REM (rare earth metals): 0.0003 - 0.01%, at least one of Ti: 0.002 - 0.1% and Nb: 0.002 - 0.1%, with Ceq which is defined by the following Equation (1) being in the range of 0.45 - 0.63, with the metallurgical structure being a mixed structure of ferrite + bainite: Ceq = C + Si / 24 + Mn / 6 + Cr + Mo / 5 + Ni + Cu / 15
wherein the symbol for each element in Equation (1) indicates the number expressing the mass percent of the element.