  We have performed electron spin resonance measurements over a wide frequency
and magnetic field range on a single crystal of the S=1 quasi-one-dimensional
Heisenberg antiferromagnet $\rm Ni(C_5H_{14}N_2)_2N_3(PF_6)$. We observed
gapped excitation branches above the critical field ($H_{\rm c}$) where the
Haldane gap closes. These branches are analyzed by a phenomenological
field-theory using complex-field $\phi^{4}$ model. A satisfactory agreement
between experiment and theory is obtained.
