Soft X-ray spectra from the plasma of argon gas jet, irradiated by an intense femtosecond Ti:sapphire laser, were obtained. Spectral lines in the range of 30 - 400Å were recorded using a flat field XUV spectrograph and a thinned backside illuminated charge-coupled device. The gas density profile of the gas jet was measured using the focusing method. The gas density in the interaction region and the trigger time of the gas injection to synchronize with laser pulse were measured. With measured expansion angles of gas jet, we estimated the mean cluster size in the gas jet expansion. Argon spectral lines up to F-like ions were observed with the backing pressure larger than 2 bar, corresponding to the gas density at the interaction region of $2×10^{18}/㎤$. Since the optical field ionization can generate only up to Na-like agron ions, additional ionization mechanisms, such as electron-ion collisional ionization, may contribute to the higher charged ions, possibly due to the formation of clusters, which will be investigated further in the future work.
