A pump assembly including a housing, a pump unit having a low pressure portion and a high pressure portion and accommodated in a pump unit receiving portion which is formed in the housing, a suction passage, a low pressure chamber communicated with the suction passage and the low pressure portion, and a discharge passage, wherein when the pump unit is accommodated in the pump unit receiving portion, the low pressure chamber is formed between the pump unit and a wall surface of the housing which defines the pump unit receiving portion, and the discharge passage is formed by a communication pipe which extends through the low pressure chamber and communicates the housing and the high pressure portion of the pump unit with each other.