The influence of the demagnetization effect on the distribution of internal magnetic fields for the cylindrical and rectangular box samples is considered. The boundary integral method is used to compute the space distribution of the magnetic field. On the basis of numerical simulations we compute the distribution of the diagonal component of the demagnetization tensor in the samples. The corresponding mathematical expectation and dispersion of this distribution are estimated. The consideration of the diagonal terms of the demagnetizing tensor is sufficient for certain field geometry in the experimental set-up. The results of the calculations are used in the analysis of the μSR data obtained for high- T c superconductors. For these compounds it is shown, that the correction to the penetration depth value related to the demagnetization broadening of the μSR frequency line may reach 7.5% in rectangular box samples.