A new driving system of an insulated probe is proposed and compared in Ar RF plasma with the conventional insulated probe system using an operational amplifier for probe current detection. In this system, the probe is driven by a low-outputimpedance driver through a transmission line as long as a half-wavelength of the fundamental component of the RF, and the probe current is detected at the midpoint of this transmission line as a voltage signal. It was shown that measurement almost equivalent to that using an operational amplifier is possible with the use of a transmission line. The advantage of this system is that the flexibility of geometric design of the probe system is greatly improved without the parasitic effect due to the increase of stray capacitance.