The invention relates to a heat exchanger for exchanging heat between a first (HP) and a second (BP) fluids. According to the invention, said exchanger comprises a plurality of modules (15,15a,15b,15c) respectively including an upper plate (17) and a lower plate (19) assembled so as to define, between said plates (17,19), an inner cavity (21) forming a first circulating channel for the first fluid (HP), wherein said exchanger further comprises a housing (3) in which said modules (15,15a,15b,15c) are assembled in order to form said exchanger while defining spaces (33) between said modules, said spaces forming second circulation channels for the second fluid (BP).