An airline ticket sales system for selling an airline ticket in combination with an outbound flight and an inbound flight, the airline ticket sales system comprising: a stock recording unit configured to retain a stock of flights; and a destination candidate extraction unit configured to: receive an input of at least one condition from a user via an information processing terminal, information about a destination airport being excluded from the condition; acquire, from the stock recording unit, information about outbound candidate flights and inbound candidate flights satisfying the condition; extract, as candidate airports, a predetermined number from airports common to arrival airports of the outbound candidate flights and departure airports of the inbound candidate flights; perform application and payment processing for the user without an ultimate destination having been determined yet; select, based on a predetermined criterion and at a predetermined timing after the application and payment and before a departure date, one from round-trip candidate flights composed of combinations of the outbound candidate flights and the inbound candidate flights among outbound departure airports specified by the condition and the respective candidate airports; and regard the selected one as a selected round-trip flight, wherein in selecting the selected round-trip flight, the destination candidate extraction unit is configured to assign a weight to and to weight each of the round-trip candidate flights, and wherein further the weight is assigned based on multiplication of a per-airport weight and an in-airport per-round-trip weight, the per-airport weight being set based on a stock quantity for each of the candidate airports corresponding to the round-trip candidate flights, the in-airport per-round-trip weight being set based on a stock quantity for each of the round-trip candidate flights to be sold in the candidate airports, and wherein the destination candidate extraction unit is further configured to determine the selected flight by drawing based on probabilities dependent on the weights. a stock recording unit configured to retain a stock of flights; and a destination candidate extraction unit configured to: receive an input of at least one condition from a user via an information processing terminal, information about a destination airport being excluded from the condition; acquire, from the stock recording unit, information about outbound candidate flights and inbound candidate flights satisfying the condition; extract, as candidate airports, a predetermined number from airports common to arrival airports of the outbound candidate flights and departure airports of the inbound candidate flights; perform application and payment processing for the user without an ultimate destination having been determined yet; select, based on a predetermined criterion and at a predetermined timing after the application and payment and before a departure date, one from round-trip candidate flights composed of combinations of the outbound candidate flights and the inbound candidate flights among outbound departure airports specified by the condition and the respective candidate airports; and regard the selected one as a selected round-trip flight, receive an input of at least one condition from a user via an information processing terminal, information about a destination airport being excluded from the condition; acquire, from the stock recording unit, information about outbound candidate flights and inbound candidate flights satisfying the condition; extract, as candidate airports, a predetermined number from airports common to arrival airports of the outbound candidate flights and departure airports of the inbound candidate flights; perform application and payment processing for the user without an ultimate destination having been determined yet; select, based on a predetermined criterion and at a predetermined timing after the application and payment and before a departure date, one from round-trip candidate flights composed of combinations of the outbound candidate flights and the inbound candidate flights among outbound departure airports specified by the condition and the respective candidate airports; and regard the selected one as a selected round-trip flight, wherein in selecting the selected round-trip flight, the destination candidate extraction unit is configured to assign a weight to and to weight each of the round-trip candidate flights, and wherein further the weight is assigned based on multiplication of a per-airport weight and an in-airport per-round-trip weight, the per-airport weight being set based on a stock quantity for each of the candidate airports corresponding to the round-trip candidate flights, the in-airport per-round-trip weight being set based on a stock quantity for each of the round-trip candidate flights to be sold in the candidate airports, and wherein the destination candidate extraction unit is further configured to determine the selected flight by drawing based on probabilities dependent on the weights.