A laser interferometer gravitational wave detector requires an ultrahigh vacuum in the tubes in which the laser beams pass. TAMA300, involves two 300-m vacuum tubes, and is kept at 2 X 10(-6) Pa of vacuum pressure so as to reduce scattering effects due to residual-gas molecules. The sensitivity attained so far is 2 X 10(-18) M/rootHz around 1 kHz. By introducing a Xe gas into the entire system of TAMA300, we directly observed the residual-gas effect on the sensitivity. It was found that Xe-gas pressure of 0.03 Pa induces an increase in the mirror displacement noise of 3 X 10(-18) m/rootHz. This noise level is,consistent with a calculated optical fluctuation of the laser beam due to residual-gas scattering.