We present the results of monitoring the galactic black hole candidate GX 3394 with the Monitor of Allsky X-ray Image (MAXI) /Gas Slit Camera in the high/soft state during an outburst in 2010. All of the spectra throughout the 8-month period were well reproduced with a model consisting of multi-color disk emission and its Comptonization component, whose fraction is ≤25% in the total flux. In spite of the flux variability over a factor of 3, the innermost disk radius is constant at Rin = 61±2 km for an inclination angle of i = 46° and a distance of d = 8 kpc. This Rin value is consistent with those of past measurementswith Tenma in the high/soft state. Assuming that the disk extends to the innermost stable circular orbit of a non-spinning black hole, we estimate the black hole mass to be M = 6.8±0.2M for i = 46° and d = 8 kpc, which is consistent with that estimated from the Suzaku observation of the previous low/hard state. Further combined with the mass function, we obtained a mass constraint of 4.3M⊙ <M < 13.3M⊙ for the allowed range of d = 6-15 kpc and i < 60°. We also discuss the spin parameter of the black hole in GX 339-4 by applying relativistic accretion disk models to the Swift/XRT data. © 2011. Astronomical Society of Japan.