A pump unit (10), comprising: a body (13) provided with an inlet (11) and an outlet (12) for a fluid； and a group of pumps composed of a plurality of micropumps (15) disposed in the body (13), to allow a fluid entering through the inlet (11) to exit from the outlet (12), in which the group of pumps includes a micropump (15) placed in a more upstream position (m1) in a serial state, a micropump (15) placed in a more downstream position (m4) in the serial state, and a micropump (15 ) placed in a central position (m2, m3) in the serial state, the body (13) includes a flow passage with direct inlet connection (41) that directly connects a suction port (31A) of the micropump (15 ) in a more upstream position (m1) with the inlet (11), a flow passage with direct outlet connection (42) that directly connects a discharge port (31B) of the micropump (15) in a more current position below (m4) with the outlet (12), and a flow passage formation mechanism that connects the micropumps (15) q They constitute the group of pumps, characterized in that the flow passage formation mechanism is switchable between the serial state in which the micropump (15) in a position more upstream (m1), the micropump (15) in a central position (m2, m3) and the micropump (15) in a more downstream position (m4) are connected in this order and a parallel state in which a branched passage is formed that connects between a suction port (31A) of the micropump (15) in a central position (m2, m3) or more downstream (m1) and the inlet (11) and a confluent passage is formed connecting between a discharge port of the micropump (15) in a position more upstream (m1) or central (m2, m3) and the outlet (12), and the pump unit further comprises a flow passage formation control part (97) for controlling the flow passage formation mechanism , and the flow step formation mechanism further includes a first step formation means of flow (52A-54A, 52X-54X) that allows the suction ports (31A) of the micropumps (15) to be co
