  A coherently oscillating scalar field, an axion as an example, is known to
behave as a cold dark matter. The arguments were usually made in the Newtonian
context. Ratra proved the case in relativistic context using the synchronous
gauge. In this paper we present another proof based on a more suitable gauge
choice, the uniform-curvature gauge, which fits the problem. By a proper time
averaging the perturbed oscillating scalar field behaves as a cold dark matter
on the relevant scales including the superhorizon scale.
