NOVELTY - An exothermic substrate (1) has a metal board (2) comprising iron-chromium-aluminum type stainless steel plate, and a resistor (6) positioned through an electric-insulation layer (3) on the metal board. The electric-insulation layer includes an underlying insulating layer and a surface insulating layer. The underlying insulating layer comprises an aluminum oxide film having thickness of 0.3-1 mu m and central-line average roughness (Ra) of 300-500 nm. The surface insulating layer comprises inorganic oxide film having central-line average roughness of 200 nm or less.    USE - Exothermic substrate is used for microreactor (claimed) for performing endothermic reaction to produce hydrogen.    ADVANTAGE - The microreactor comprising exothermic substrate having resistor with uniform thickness has excellent reliability, and provides high catalytic reaction efficiency. The generation of cracks and peeling-off of electric-insulation layer due to heat are suppressed.    DETAILED DESCRIPTION - An exothermic substrate has a metal board comprising iron-chromium-aluminum type stainless steel plate, and a resistor positioned through an electric-insulation layer on the metal board. The electric-insulation layer includes an underlying insulating layer and a surface insulating layer laminated from the metal board side. The underlying insulating layer comprises an aluminum oxide film having thickness of 0.3-1 mu m and central-line average roughness (Ra) of 300-500 nm. The surface insulating layer comprises inorganic oxide film having thickness of 0.3-1 mu m and central-line average roughness of 200 nm or less. INDEPENDENT CLAIMS are included for the following:    (1) manufacture of exothermic substrate, which involves heating metal board containing iron-chromium-aluminum type stainless steel plate, forming aluminum oxide film on the surface of metal board, setting aluminum oxide film as underlying insulating layer on metal board, coating perhydro polysilazane solution on underlying insulating layer, forming surface insulating layer in which underlying insulating layer and surface insulating layer are composed to form electric-insulating layer, and positioning resistor on the electric-insulating layer; and    (2) microreactor, which has a metal microreactor main component in which a catalyst is supported inside, and the exothermic substrate welded to surface(s) of microreactor main component.    DESCRIPTION OF DRAWING(S) - The drawing shows the top view of top view of exothermic substrate.    Exothermic substrate (1)    Metal board (2)    Electric-insulation layer (3)    Resistor (6)    Electrode (6a)