Hydrogen production by photosynthetic bacteria provides an efficient energy conversion method under low light intensity. However, under strong illumination, such as midday sunlight, the efficiency drops. This prevents the method from being applied industrially. To overcome this problem, we examined a method to thin out the excessive illumination. Light was given intermittently to reduce the total energy flux. The on/off ratio was set at 1/1 throughout the study, so that the time average of the light energy flux became half the continuous illumination. By keeping the time-average light flux constant (0.6 kW.m-2), the effects of the cycle period were examined in the range of hours to seconds. The hydrogen production rate was greatly affected by the cycle period, but cell growth and substrate consumption rates remained almost constant. The 30-min light/dark cycle (30 min on and 30 min off) provided the highest rate of hydrogen production (22 L.m-2.24 h-1). At the shorter cycles, the rate decreased except that there was a suboptimum at about 40 s. Under excessive light intensity (1.2 kW.m-2), the light-to-hydrogen conversion efficiency was greatly enhanced. The hydrogen production rate during the 30-min cycle was twice as high as during a 12-h cycle under the same conditions.