Low-temperature specific heat and magnetization measurements have been performed on the quantum spin system NH4CuCl3 which has magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. It is found that the present system undergoes three-dimensional magnetic ordering at TN = 1.3 K. It is suggested that one-quarter of the spins are ordered below TN, and the remaining spins are in the singlet state. The small magnetic entropy at TN indicates that the phase transition occurs under the condition of a well-developed spin correlation around TN. The field dependence of the three-dimensional ordering is also investigated on the basis of magnetization measurements. Based on this evidence, a magnetic phase diagram for NH4CuCl3 is proposed.