In this paper, the characteristics of microdischarge phenomena in a plasma device with an auxiliary electrode located on the center of the coplanar gap were investigated. The microdischarge showed three discharge modes in accordance with the auxiliary pulse voltages, and its luminous efficacy was improved when it operated in the efficient mode. To understand the mechanisms of that improvement of efficiency, a wall-charge measurement system was proposed and applied to measure the wall-charge behavior, which is a key for the diagnosis of discharge characteristics. For the efficient mode, measurement of wall-charge behavior indicated the presence of the three high efficiency factors, namely, long-gap discharge, current reduction, and priming effect. For the inefficient mode, however, the measured wall-charge behavior indicated that strong short-gap discharges were generated between sustain and auxiliary electrodes, and then, the high efficient factors were not observed. Additionally, in order to operate the device in the efficient mode, the tendency of the mode transition was investigated in terms of panel specifications, such as barrier rib height and coplanar gap. The results indicate that the transition voltage between the efficient and inefficient modes increased when the barrier rib height increased, and it showed the tendency resembling the Paschen curve with various coplanar gaps.