PURPOSE: To provide a gerotor pump of inexpensive manufacturing and assemblying costs and capable of reducing pressure pulsation to a lower value than the same kind of conventional pumps.

CONSTITUTION: A plurality of pump chambers of variable capacity are formed between an outer gear rotor 48 and an inner gear rotor 46. There is a gap 60 between a ring wall 44 of the inlet end cap and the outer gear rotor 48 which is journalled solely to the bearing surface 66 of a bearing pad 64 formed near an outlet pocket 58. Therefore the outer gear rotor 48 can freely float besides the bearing surface 66. Because a pump chamber near the outlet pocket 58 is of high pressure for its capacity being ensmalled, fluid leakage between mutual pump chambers is prevented by pressing the outer gear rotor 48 against the bearing surface 66 and strengthing the engagement of the inner gear with the outer gear on the opposite side.