an air pump is provided with a head , a cylinder , a valve chamber , a branch duct , an air discharging hole , and an elastic member . the cylinder comprises a first air chamber , and a second air chamber smaller in the cross - sectional area than the first air chamber . when air pressure in an inflatable object being inflated by the air pump has reached a certain pressure level , a displacement member of the valve member is caused to displace so as to allow the air in the first air chamber to be released into the atmosphere via the branch duct and the air discharging hole . in the meantime , the pumping action is automatically switched to the second air chamber through which air is continuously pumped into the inflatable object .