An object is to improve detection accuracy of unfair use when a task system is used by using a mobile terminal.There is provided a method for detecting unfair use including: recording a plurality of operation feature quantities which are calculated from an operation log of a user and weights for the plurality of operation feature quantities of the user which are calculated by using a plurality of operation feature quantities of other users, obtaining an operation log of the user of a terminal device, calculating a plurality of operation feature quantities from the operation log, calculating similarity between the calculated plurality of operation feature quantities and the recorded plurality of operation feature quantities of the user in advance, by using the weights for the plurality of operation feature quantities, and detecting unfair use of the terminal device.