  We performed $^{31}$P-NMR measurements on LaFe(As$_{1-x}$P$_{x}$)O to
investigate the relationship between antiferromagnetism and superconductivity.
The antiferromagnetic (AFM) ordering temperature $T_{\rm N}$ and the moment
$\mu_{\rm ord}$ are continuously suppressed with increasing P content $x$ and
disappear at $x = 0.3$ where bulk superconductivity appears. At this
superconducting $x = 0.3$, quantum critical AFM fluctuations are observed,
indicative of the intimate relationship between superconductivity and
low-energy AFM fluctuations associated with the quantum-critical point in
LaFe(As$_{1-x}$P$_{x}$)O. The relationship is similar to those observed in
other isovalent-substitution systems, e.g., BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$
and SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, with the "122" structure. Moreover,
the AFM order reappears with further P substitution ($x > 0.4$). The variation
of the ground state with respect to the P substitution is considered to be
linked to the change in the band character of Fe-3$d$ orbitals around the Fermi
level.
