An electronic commerce server (10) including a time adjustment information memory unit (10b), an irregular day information comparison circuit (43), and an irregular day notification circuit (44). The time adjustment information memory unit (10b) records information on time adjustment. The irregular day information comparison circuit (43) compares the transaction available period and the time adjustment information. The irregular day notification circuit (44) notifies the occurrence of a day on which time adjustment is performed. The electronic commerce server (10) checks whether there is a region where time adjustment is performed during the transaction available period. If a start-date or end data of Daylight Saving Time occurs, that day is checked by the irregular day information comparison circuit (43). The irregular day notification circuit (44) transmits the occurrence of a time adjustment day to the computer (1a) for the seller. The seller changes that transaction available period, and the buyer learns the fact that an irregular day, that is the start-date or end-date, occurs during the transaction available period and how much the transaction available period will differ from the current clock time.