PURPOSE: To prevent abrasion of the specified position and early breakage of the diaphragm of a diaphragm air pump which is used when air is supplied to a purifying chamber or a water tank for cultured fishes and living fishes.

CONSTITUTION: A diaphragm 5 which an expanable and contractible part 5D composing a wall thickness part 5D1 and a thin wall part 5D2 is formed between a boss part 5C and an outer circumferential part 5B arranged on nearly center part, is put and held between both end parts of a driving rod 2 which is performed reciprocating movement by two diaphragm push plates 6, 6', and the diaphragm 5 is thereby, installed in a solid structure against extension and bending.