  Let $L f(x):=-\frac{d^2}{dx^2}f(x)-\frac{ r}{x}\frac{d}{dx}f(x),\quad x>0,
r>0$ be the Bessel operator on $((0,\infty), |\cdot|, x^rdx)$. In this paper,
we prove the sharp weak type $(1,1)$ estimate for the imaginary power
$L^{i\alpha}, \alpha\in \mathbb R$, of the Bessel operator.
