The present invention relates to a door closer comprising a cylinder body and a telescopic rod, wherein a one-way air intake valve is provided in the cylinder body to take in air in one way in an internal cavity of the cylinder body when the door is opened. The internal cavity of the cylinder body is divided into two automatically pressurizeable and automatically compensatable air cavities. When the door is opened or closed, the air inside the internal cavity of the cylinder body flows cyclically in the two air cavities. The door closer ensures air substantially flows cyclically in a cylinder body, reduces pollution caused by dusts in air into and out of the cylinder body to the lubricant oil in the cylinder wall, guarantees the lubricating effect of the lubricant oil, enables the door closer to operate normally and effectively prolong the service life of the door closer.