To provide a triple piston pump unit in which a direction of a line of an air chamber and a pressure regulation valve to a pump unit body can be selected.

A discharge side space 50 is formed inside a cylinder tip case 20, liquid is discharged from each discharge valve 36, and an opening 52 is provided in a junction surface 76 at an upper part of the cylinder tip case 20. An assisting unit installation body 54 is provided with an air chamber 60 and a pressure regulation valve 64, a passage space 56 is formed inside the assisting unit installation body 54 to be communicated with the air chamber 60 and the pressure regulation valve 64, and an opening 58 is provided in a junction surface 77 of the assisting unit installation body 54. The assisting unit installation body 54 is set in a desired direction, the junction surfaces 76, 77 are put to each other to set the openings 52, 58 on each other, and the assisting unit installation body 54 is fixed onto the cylinder case 20 by a stud bolt 74 and a bolt 80.