Disclosed is a heat exchanger that can more efficiently transfer heat between a heat-exchange fluid and an object with which heat is to be exchanged. A heat exchanger (1) can transfer heat between a heat-exchange fluid flowing through flow paths (R1) and a fluid with which heat is to be exchanged flowing through other flow paths (R2) by means of the flow path structure member (10) (a first metal sheet (11) and a second metal sheet (12)) in which the flow paths (R1 and R2) are formed. The flow paths (R1 and R2) are formed so that the side surfaces thereof are not straight and so that the depths thereof change along the flow direction.