Provided is a heat exchanger that can produce an upflow of coolant even when structured such that coolant is not directly supplied to a lower part of a space that generates a coolant upflow inside a header collecting pipe. Also provided is an air conditioning device. A plurality of flat, multi-hole pipes (21b) are connected at different heights to a first inner space (23a) of a folding-back header collecting pipe (23) of an outside heat exchanger (20). The first inner space (23a) has a loop structure that includes a first partitioning plate (51), a first inflow hole (41x) that is for causing coolant to rise into a first outflow space (51a), and a first lower passageway (51y) that causes coolant from the first upper passageway (51x) to descend through a first loop space (51b) and guides the coolant back to the first outflow space (51a). One end of the flat, multi-hole pipes (21b) is connected to either the first outflow space (51a) or the first loop space (51b). Connective piping (24) is connected to a space that is below the first loop space (51b) across a first rectifying plate (41).