An artificial ventilation apparatus includes: a connecting portion which is connected to a respiratory system of a patient; an inspiratory circuit which is a flow path for flowing a gas from a ventilator to the connecting portion; an expiratory circuit which is a flow path for guiding a gas exhausted from the connecting portion to an exhaust portion of the ventilator; an expiratory valve which blocks a flow of a gas from the exhaust portion toward the connecting portion; a carbon dioxide concentration sensor which is disposed in a circuit that is provided at a downstream side of the expiratory valve and which detects a carbon dioxide concentration; and an alarm outputting unit which outputs an alarm based on an output of the carbon dioxide concentration sensor.