A parking reservation system for a parking lot provided with a plurality of parking spaces, comprising parking spaces for reservation to which reservation of user with reservation is set and is also available for user without reservation; and spare parking spaces which the user with reservation can use instead of the parking spaces for reservation when the parking spaces for reservation to which reservation of the user with reservation is set is used by the user without reservation, wherein a spare number setting section is provided for setting number of the spare parking spaces by using usage behavior relation information in relation to usage behavior at the parking lot of the user at least either of the user without reservation or the user with reservation.