A medium processing apparatus (3) includes a liquid applier (31), a post-processing device (32), a liquid storage unit (44, 47), a mover (35), and a liquid supplier (45, 46). The liquid applier (31) applies liquid to at least one medium (P). The post-processing device (32) performs a given process on a bundle of media (Pb). The liquid storage unit (44, 47) stores the liquid to be supplied to the liquid applier (31). The mover (35) causes the liquid applier (31) to move in a reciprocating direction. The liquid supplier (45, 46) supplies the liquid from the liquid storage unit (44, 47) to the liquid applier (31) and includes an extensible member (453) having an elastic cylindrical part on at least a part of the liquid supplier (45, 46). The extensible member (453) extends in the reciprocating direction of the mover (35).