We developed a computer-based system for controlling water vapor conditions (i.e., humidity) using a two-flow method in which streams of humidified and dehumidified air were combined in an acrylic container. The flow rate of each stream was independently controlled to adjust relative humidity (RH). In this system, humidification from 15 to 90 % RH and dehumidification from 90 to 15 % RH at an air temperature (AT) of 25 °C were properly operated with short time constants of 4.3 and 10 min, respectively. Tetranychus urticae egg hatchability was then examined at 20-95 % RH and 25 °C AT. The coefficients of variation of RH were low (0.3-1.5 %). Egg hatchability in a polystyrene Petri dish was lower at 20 % RH than at 70-95 % RH. A delay in hatching was also observed at 70 % RH for eggs tested on a leaf disk placed on water-soaked cotton; this delay was attributed to the AT being 1.4 °C lower on the leaf surface than on the inner surface of the dish. Our system is expected to be useful for further examination of ecological and behavioral responses in pest mites and for developing novel physical control measures using water vapor.