There are two ways of becoming listed firms for unlisted firms. One is initial public offering (IPO) and the other is backdoor listing(BL). IPO has stick listing requirement and procedure for protecting new investor, but BL don't have stick listing procedure and don’t require compare with IPO because it's trade between shareholders of listing firm and shareholder of unlisted firm. A regulatory agency permit BL because of market activation. But large shareholder of unlisted firm abuse BL as a method of personal gain, as a result, bona-fide small investors are affected. So a regulatory agency solve negative aspects of BL and amend relative regulation consistently for market activation. A regulatory agency strengthened listing requirement, supervision, disclosure through ‘Backdoor Listing The Regulatory Change’ in 2006. The backdoor listing company must disclose a business report separately pro forma information and performance information of unlisted business segment which they merge as backdoor listing in the two years since company listed. This is system for damage minimization of BL firms investor according to overestimate of unlisted shares which it asses value based on past performance and pro forma information. Choi and Choi(2014) examines the difference of the earnings quality of the BL firms the before and after the regulations strengthened in 2006. In a result, they reported that the earnings quality of the BL firms the before and after regulations strengthened is not significantly different. But this paper investigates effectiveness of strengthened disclosure in ‘Backdoor Listing The Regulatory Change’ in 2006. For effectiveness investigation, we analyzed whether external stakeholder recognize discriminately overestimation or underestimation of pro forma information. Research findings are as follows. First, optimistic pro forma information(net income, sales and operating income) of BL unlisted firm are positively associated with the cost of capital (COC), respectively. In other words, capital market participants recognize negatively about overestimation of pro forma income information. The second, investors negatively recognize when the accuracy of pro forma income information is low. The third, we re-examinate hypothesis about firms which overestimate pro forma income, the result don't different qualitatively from main hypothesis investigation result. Recently, it is regarded that BL by SPAC(special purpose acquisition company). This study will help to make a system which profits of general investors protected at a time when BL is expected to vitalize of equity market.