According to one aspect of the present invention, a method for diagnosing failure in a pressure gauge of a hydrogen filling system includes filling a fuel cell vehicle powered by hydrogen fuel with the hydrogen fuel from an accumulator, in which the hydrogen fuel is accumulated, via a dispenser; and acquiring pressure values measured by a plurality of pressure gauges disposed at different positions in a flow passage of the hydrogen fuel between the accumulator and an outlet of the dispenser at timing when a flow rate of the hydrogen fuel to be filled at a stage close to an end of the filling becomes a threshold value or less, determining whether or not a deviation between the pressure values is within a threshold value on the basis of acquired pressure values, and outputting a determination result.