A four-cylinder diaphragm pump includes a pump body having four pump chambers, and a drive mechanism that expands and contracts the four pump chambers with a predetermined phase difference, in which the pump body includes a first diaphragm having two diaphragm portions on a same plane, and a second diaphragm having two diaphragm portions on a same plane disposed to be located to be parallel to or coplanar with the plane of the first diaphragm, each of the diaphragm portions of the first diaphragm and the second diaphragm constitute a portion of a different pump chamber, and the drive mechanism is configured to move the diaphragm portions of the first diaphragm and the second diaphragm forward or backward with respect to the corresponding pump chambers with a predetermined phase difference.