We proved sharp estimates for the weak type $(1,1)$ norm of such operators. The obtained result is optimal i.e. the lower bounds are proportional to the upper bounds. Applying our result to the standard Laplace operator on ${\bf R}^N$ we get $\|\Delta_N^{is}\|\sim (1+|s|)^{N/2}$. This estimate is more precise than the estimate which we obtain in virtue of the classical Hörmander multiplier theorem.