A heat treating oven incorporating at least one oven section (8, 8a, 8b), built in a modular system, and where the inner sides (13, 17) of the oven walls at least partly are provided with reflecting material and support a number of infra-red radiating tubes (15) connectable to a current source, and which oven section is equipped with supply air terminal devices (10, 14, 18). In order to make the oven more flexible this oven section has at least one heating unit (11) connected to the supply air terminal devices (10, 14, 18) and adapted to be optionally switched in for heating the supply air to an optional temperature, in order to impart upon the oven section a combination of heating by optional proportions of radiation heat caused by the infra-red tubes and convection heat caused by the heated supply air.