  A two-phase argon detector has high discrimination power between electron
recoil and nuclear recoil events based on the pulse shape discrimination and
the ionization/scintillation ratio (S2/S1). This character is very suitable for
the dark matter search to establish the low background experiment. However, the
basic properties of S2/S1 of argon are not well known, as compared with xenon.
We report the evaluation of S2/S1 properties with a two-phase detector at
drift-fields of 0.2-3.0 kV/cm. Finally, the discrimination power against
electron recoil background of S2/S1 is discussed.
