The underground and subway space, which is one of main public transportations and representative underground area in Seoul, causing interior air pollution and environmental degradation, could threaten citizens` health and aesthetic. The purpose of the research is to estimate willingness to pay for horticultural metro-farm facilities and air quality improvement by using onsite DBCV in metropolitan Seoul. The in-person 539 survey results show that ``aesthetic emotional stability`` gets the highest mark, followed by ``air quality improvement``, ``scenery``, ``environmental awareness``, etc. The WTP estimated is about ￦2,382 per household and the benefit expanded of metro-farm introduction is calculated to be ￦234.1 billion (WTP ￦2,381.5/3years/1st× 4,117,000 households×response rate 79.6%), and then the following net benefit is ￦148.5 billion assuming that total cost is ￦85.7 billion (ten metro-farm facilities for each one of 298 stations in Seoul), and BC ratio is about 2.73. Hence, the result shows that introduction of horticultural metro-farm facilities to improve the underground environment in Seoul will produce a variety of beneficiary effects and their distribution and expansion is expectable.