We propose a novel method for producing a high repetitive frequency short pulse to use a halved confocal of an unstable concave and convex resonator on very high frequency (VHF) discharge excitation slab type carbon dioxide laser. This method utilizes a full reflection concave mirror with a function of variable curvature, which can be controlled using a piezo electric device. Generally, slab type lasers are modulated directly by varying pulse voltage. However, there is a fluctuation of plasma at the transition of the pulsation. Consequently, the repetitive frequency is less than 10 kHz, the minimum pulse width is 1μs. On the other hand, there is no disturbance of the plasma, because the power source is not modulated in our method. And the mechanical constraint is also little, because it are used a piezo electric device. So, pulse shortening of the high repetition is possible. The pulse shape was nicely reproduced when we calculate to use the three-level rate-equation. The mechanism of the method is interpreted to attribute the changed the ejection efficiency to the vibrated along the radius of curvature of the mirror. Good agreements between the observation and the calculation are also obtained in the peak power and the pulse width. In this paper, we compared the measurement with the calculation. Then, the possibility of high repetition oscillation and pulse shortening was examined by this method. From the results, the repetitive frequency confirmed between 1.4 kHz and 170 kHz. The pulse width can be realized to shortest 160 ns.