  The analyzing power of ${\vec p}p\to pp{\pi}^0$ reaction has been measured at
the beam energy of 390 MeV. The missing mass technique of final protons has
been applied to identify the $\pi^0$ production event. The dependences of the
analyzing power on the pion emission-angle and the relative momentum of the
protons have beenobtained. The angular dependence could be decomposed by the
Legendre polynomial and the relative contribution of the $P_{21}$ to $P_{11}$
function is less than 20%. The P-state amplitude is found to be the dominant
component of the $\pi$ production near the threshold. The momentum dependence
of the analyzing power has been studied to obtain the information about the
pion production mechanism. It has been deduced that the pion production due to
the long range interaction plays an important role in the momentum dependence
of the P-state amplitude.
