The Tendering and Bidding Law" states that the bidder should not compete bidding through quoted price lower the cost, but it does not ascertain the meaning of the cost of building product. It is the purpose of this paper to ascertain the meaning of the building product cost. The calculation bases of both the social average product cost and of the enterprise individual product cost are analyzed and they are compared with each other. The authors point out that for some enterprise there is individual product cost lower the social average product cost. The function of design mechanism of bidding is revealing "private information" on "the lowest price". The purpose of carrying out bidding is introducing competition, reducing the cost for the owner and realizing the maximum yield. Both indicate the cost of building product, mentioned in "The Tendering and Bidding Law" should be the enterprise individual product cost.