For efficient control of fuel cells, highly sensitive measurement of the concentration of hydrogen gas (H2) under high humidity is required. In this study, we fabricated the ball SAW sensor with sputtered Pt-coated Zno film as a sensitive film and evaluated the response to H2 of the concentration in the range of 200 ppm to 20 ppm. The thicknesses of ZnO and Pt films are 200 nm and 5 nm, respectively. The sensor succeeded in detecting 20 ppm H2 by the amplitude change of -0.79 dB. After the sensor was wetted by water, the response was measured again. As a result, 20 ppm H2 was detected by 0.4 dB. We calculated the detection limit (DL) defined as the concentration corresponding to the signal to noise ratio of 3. Since DLs before and after wetted was 0.47 ppm and 0.27 ppm, respectively, the sensitive film was not deteriorated by wetting. In conclusion, it was shown that the ball SAW sensor with Ptcoated ZnO film was useful for humidity-proof H2 sensor with sub-ppm DL.