  The impurity-induced antiferromagnetic ordering of the doped Haldane magnet
Pb(Ni{1-x}Mg{x})2V2O8 (0 < x <0.06) was studied by electron spin resonance
(ESR) on ceramic samples in the frequency range 9-110 GHz. Below the N\'{e}el
temperature a transformation of the ESR spectrum was found, indicating an
antiferromagnetic resonance mode of spin precession. The excitation gap of the
spin-wave spectrum increases with increasing Mg-concentration $x$ in the same
manner as the N\'{e}el temperature, reaching its maximum value of 80 GHz at x >
0.04. At small concentrations x < 0.02 the signals of antiferromagnetic
resonance were found to coexist with the signal of the paramagnetic resonance
indicating a microscopic separation of the magnetic phases.
