The investigation of processes of internal-stress relaxation in superplastic eutectic alloy Sn-38wt.% Pb during ageing at room temperature is performed. The conclusion that the stress level is, at least, higher than yield stress of the material at room temperature is made on the ground of data of linear-sizes’ changes. These internal stresses remain in the material during sufficiently long term. The metastability of phase state of the alloy is considered as one of the main reason of internal-stress presence in the system. As found, both the optimal stress for superplasticity effect revealing and the observed elongation to failure are changing significantly during natural aging process of the alloy and because of preliminary annealing at $\cong$ 70°C. This is the evidence of dependence of superplastic properties, in particular, on the internal-stress level in the material.