A gas level display controller includes a mass computation module, a reference value setting module, and a display control module. The mass computation module obtains a pressure and a temperature of compressed hydrogen stored in a hydrogen tank and calculates a mass of the compressed hydrogen in the hydrogen tank, for example, according to a known gas state equation. The reference value setting module sets a reference value, which is used for display of a remaining amount of the compressed hydrogen in the hydrogen tank on a display device, to the mass of the compressed hydrogen calculated by the mass computation module on the occasion of filling the compressed hydrogen into the hydrogen tank to limitation of the hydrogen filling. The display control module calculates a ratio of the mass of the compressed hydrogen to the reference value and makes the remaining amount of the compressed hydrogen in the hydrogen tank displayed on the display device, based on the calculated ratio. This arrangement enables a symbol ‘FULL’ to be reliably displayed on a display device on the occasion of filling a compressed gas into a tank to limitation of the gas filling.