The pump has a casing accommodating a piston and a driving part. The casing has a first casing member having a driving part retaining portion retaining the driving part, a second casing member fixedly stacked on the first casing member in the reciprocating direction of the piston, and a cylindrical pump chamber peripheral wall member disposed around a head of the piston. The second casing member has an end wall portion extending in a transverse direction substantially perpendicular to the reciprocating direction. A pump chamber, a delivery chamber, and a buffer chamber are defined between the first casing member and the end wall portion of the second casing member. The pump chamber, the delivery chamber, and the buffer chamber are disposed side-by-side in the transverse direction.