We have been studying in-line gas concentration sensor units for various metal-organic(MO) gases used in electronic device manufacturing process. MO gases are low vapor pressure and are generally supplied by bubbling. At the time of bubbling supply, concentration control is performed by internal pressure control and feedback control by in-line gas concentration sensor units for concentration correction due to lowering of the liquid level inside the MO tank. We have evaluated a high sensitivity in-line gas concentration sensor that adopts ultraviolet absorption method and charge amplifier detection circuit and demonstrated that it is a unit with high response speed of 400 msec or less and high detection sensitivity. In this report, we confirmed that it is possible to measure the concentration of Zr, Hf type MO gas used for semiconductor gate insulating film.