EN
Volterra's integral equation (which arises from the first interior Fourier's problem) by spline functions of the cubic polinomials. Namely, the approximate solution of this equation is represented in the form of linear combination of spline functions, which are forming the distributions of unity within the segments [0, 2] and [0,t] respectively. The error of approximation we associate on natural way with perfectin of considered distributions of unity. The estimation of the error is given at the end of the paper.