Single crystals of a Na-Ga-Si clathrate, Na8Ga5.70Si40.30, of size 2.9 mm were grown via the evaporation of Na from a Na-Ga-Si melt with the molar ratio of Na : Ga : Si = 4 : 1 : 2 at 773 K for 21 h under an Ar atmosphere. The crystal structure was analyzed using X-ray diffraction with the model of the type-I clathrate (cubic, a = 10.3266(2) Å, space group Pm3n, no. 223). By adding Sn to a Na-Ga-Si melt (Na : Ga : Si : Sn = 6 : 1 : 2 : 1), single crystals of Na8GaxSi46−x (x = 4.94-5.52, a = 10.3020(2)-10.3210(3) Å), with the maximum size of 3.7 mm, were obtained via Na evaporation at 723-873 K. The electrical resistivities of Na8Ga5.70Si40.30 and Na8Ga4.94Si41.06 were 1.40 and 0.72 mΩ cm, respectively, at 300 K, and metallic temperature dependences of the resistivities were observed. In the Si L2,3 soft X-ray emission spectrum of Na8Ga5.70Si40.30, a weak peak originating from the lowest conduction band in the undoped Si46 was observed at an emission energy of 98 eV.