A bid-opening device 20 receives from each bidding device 10m a bidding price index γm=g(hk(IVm)) (where IVm is an initial value, hk indicates k-times processing with a one-way function h and g is a one-way function) and its identifier IDm. The received information is stored on a common bulletin board 25. Dm=hk(IVm) (where Dm=g(Dm) is generated with k set as the upper limit value K of the bidding price. The bulletin board 25 is checked for γm which matches this Dm. If no match is found, m is incremented by one, and the check for γm−Dm matching is made for each of m bidding devices. Upon completion of the matching for all the bidding devices, k is decremented by one, and a check is made for γm which matches Dm=g(hk(IVm)), and the index k for which they match is determined as the highest price bid. The bid-opening device 20 outputs that k and the identifier IDm of γm.