A lasing device providing laser medium gas continuously or intermittently from outside, comprising: an optical resonator; a gas circulation path connected to the optical resonator; a laser medium gas supply device for supplying laser medium gas to the gas circulation path or the optical resonator via a gas supply valve; a gas release pump for releasing laser medium gas from the gas circulation path of the optical resonator via a gas release valve; a gas pressure detector for detecting gas pressure of the laser medium gas in the gas circulation path or the optical resonator; a gas pressure controller for controlling the gas supply valve and the gas release valve in response to the gas pressure detected by the gas pressure detector; a blower disposed in the gas circulation path; a current detector for detecting blower driving current for the blower; a stop-time counter for counting a stop time of the lasing device; a storage for storing correlated information of the stop time with the blower driving current; and characterized in that it further comprises an open-time calculator configured to calculate an open time of the gas supply valve at a start up of the lasing device based on the correlated information of the stop time with the blower driving current from the storage, wherein, the lasing device is configured such that when said lasing device is turned on after having a stop state, the gas pressure controller is configured to - open the gas release valve in order to release the gas retaining in the gas circulation path and the optical resonator through the gas release valve, and - open the gas supply valve during the open time of the gas supply valve calculated by the open-time calculator, so that the gas supply valve is configured to let enter the laser medium gas in the piping between the laser medium gas supply device and the gas supply valve in the gas circulation path and the optical resonator that is released, together with the retaining gas, through the gas release valve opened by the gas pressure controller. an optical resonator; a gas circulation path connected to the optical resonator; a laser medium gas supply device for supplying laser medium gas to the gas circulation path or the optical resonator via a gas supply valve; a gas release pump for releasing laser medium gas from the gas circulation path of the optical resonator via a gas release valve; a gas pressure detector for detecting gas pressure of the laser medium gas in the gas circulation path or the optical resonator; a gas pressure controller for controlling the gas supply valve and the gas release valve in response to the gas pressure detected by the gas pressure detector; a blower disposed in the gas circulation path; a current detector for detecting blower driving current for the blower; a stop-time counter for counting a stop time of the lasing device; a storage for storing correlated information of the stop time with the blower driving current; and characterized in that it further comprises an open-time calculator configured to calculate an open time of the gas supply valve at a start up of the lasing device based on the correlated information of the stop time with the blower driving current from the storage, wherein, the lasing device is configured such that when said lasing device is turned on after having a stop state, the gas pressure controller is configured to - open the gas release valve in order to release the gas retaining in the gas circulation path and the optical resonator through the gas release valve, and - open the gas supply valve during the open time of the gas supply valve calculated by the open-time calculator, so that the gas supply valve is configured to let enter the laser medium gas in the piping between the laser medium gas supply device and the gas supply valve in the gas circulation path and the optical resonator that is released, together with the retaining gas, through the gas release valve opened by the gas pressure controller. - open the gas release valve in order to release the gas retaining in the gas circulation path and the optical resonator through the gas release valve, and - open the gas supply valve during the open time of the gas supply valve calculated by the open-time calculator, so that the gas supply valve is configured to let enter the laser medium gas in the piping between the laser medium gas supply device and the gas supply valve in the gas circulation path and the optical resonator that is released, together with the retaining gas, through the gas release valve opened by the gas pressure controller.