The first candidate determination means 15B is configured to determine, based on seller information 41B indicating a sale condition of a transaction target presented by each of sellers and buyer information 42B indicating a buy condition of the transaction target presented by each of buyers, first candidates C1x to be plural candidates for combinations of the sellers and the buyers establishing valid transactions of the transaction target. The second candidate selection means 16B is configured to select plural second candidates C2x to be displayed from the first candidates C1x based on similarity among the first candidates C1x.