A pump comprising: a reciprocating pumping member configured to reciprocate in a reciprocating direction; a driving part configured to reciprocate the reciprocating pumping member; and a casing accommodating the reciprocating pumping member and the driving part; the pump being configured to convey a fluid by a reciprocating motion of the reciprocating pumping member; the casing having: a first casing member having a driving part retaining portion retaining the driving part, and a buffer chamber forming portion; a second casing member fixedly stacked on the first casing member in the reciprocating direction, the second casing member having an end wall portion facing the reciprocating pumping member in the reciprocating direction and extending in a transverse direction crossing the reciprocating direction; and a cylindrical pump chamber peripheral wall portion extending in the reciprocating direction between the driving part retaining portion and the end wall portion around the reciprocating pumping member; wherein 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 and disposed side-by-side in the transverse direction crossing the reciprocating direction, the pump chamber being located inside the pump chamber peripheral wall portion and having a volumetric capacity varied by the reciprocating motion of the reciprocating pumping member, the delivery chamber being located around the pump chamber peripheral wall portion and communicating with the pump chamber through a first communication passage extending through the pump chamber peripheral wall portion in the transverse direction, and the buffer chamber being adjacent to the delivery chamber in the transverse direction and communicating with the delivery chamber through a second communication passage extending in the transverse direction, and a part of the buffer chamber being defined by the buffer chamber forming portion. a first casing member having a driving part retaining portion retaining the driving part, and a buffer chamber forming portion; a second casing member fixedly stacked on the first casing member in the reciprocating direction, the second casing member having an end wall portion facing the reciprocating pumping member in the reciprocating direction and extending in a transverse direction crossing the reciprocating direction; and a cylindrical pump chamber peripheral wall portion extending in the reciprocating direction between the driving part retaining portion and the end wall portion around the reciprocating pumping member; wherein 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 and disposed side-by-side in the transverse direction crossing the reciprocating direction, the pump chamber being located inside the pump chamber peripheral wall portion and having a volumetric capacity varied by the reciprocating motion of the reciprocating pumping member, the delivery chamber being located around the pump chamber peripheral wall portion and communicating with the pump chamber through a first communication passage extending through the pump chamber peripheral wall portion in the transverse direction, and the buffer chamber being adjacent to the delivery chamber in the transverse direction and communicating with the delivery chamber through a second communication passage extending in the transverse direction, and a part of the buffer chamber being defined by the buffer chamber forming portion.