An expansion device includes: an irradiation unit configured to irradiate a thermally-expandable sheet with light; a movement unit configured to relatively move the thermally-expandable sheet and the irradiation unit; an air-blowing unit configured to cool the irradiation unit by blowing air to the irradiation unit; and a control unit configured to execute a distension process of distending the thermally-expandable sheet by causing the irradiation unit to emit light while relatively moving the thermally-expandable sheet and the irradiation unit by the movement unit, wherein the control unit controls the air-blowing unit to stop or weaken blowing air toward the irradiation unit while the irradiation unit is caused to emit light in the distension process.