A method of reaction which comprises using a reactor which is equipped with a catalyst layer for reacting a raw-material gas feed and is capable of cooling and heating through heat exchange with the catalyst layer, wherein the reactor, in the state of comprising no catalyst layer, has a differential pressure of 50 Pa or lower when air having 0 °C is introduced into the reactor at a space velocity, in terms of normal-state velocity per unit catalyst layer volume, of 7,200 1/hr while regulating the reactor outlet pressure to 1 atm and without conducting heating with a heat medium, the catalyst layer having been divided into a plurality of reaction zones arranged from a raw-material gas inlet to a raw-material gas outlet, the multiple reaction zones having been disposed so that the porosity thereof, which refers to the proportion of that part of the catalyst-packed space which is not occupied by the catalyst, as measured at the time of catalyst packing, decreases from the raw-material gas inlet toward the raw-material gas outlet, wherein the reactor is a plate-type reactor which is equipped with a catalyst layer formed between adjacent heat-transfer plates and in which a raw-material gas feed passes through the space between the adjacent heat-transfer plates and is then discharged, a catalyst layer being formed between the adjacent heat-transfer plates, and wherein the reactor is a plate-type reactor in which the catalyst layer formed between the adjacent heat-transfer plates has been divided into a plurality of reaction zones arranged from the raw-material gas inlet toward the raw-material gas outlet, and the multiple reaction zones increase in catalyst layer thickness, in terms of thickness in the direction of the length of the space between the adjacent heat-transfer plates, from the raw-material gas inlet toward the raw-material gas outlet, the ratio of the particle diameter of the catalyst to be packed into the space between the adjacent heat-transfer plates to the minimum spacing (d) between the adjacent heat-transfer plates (D/d) satisfying 0.9>D/d>0.1, wherein the reactor is a reactor which, in the state of being equipped with a catalyst layer, has a differential pressure of 15 kPa or lower when air having 0 °C is introduced into the reactor at a space velocity, in terms of normal-state velocity per unit volume of the catalyst layer, of 1,800 1/hr while regulating the reactor outlet pressure to 1 atm and without conducting heating with a heat medium; wherein adjacent heat-transfer plates are disposed so as to be parallel with each other; and wherein "normal-state" refers to the conditions of 0°C and 1 atm, the term "thickness of the catalyst layer" refers to the average of catalyst-layer thicknesses as measured in the direction of the length of the space between heat-transfer plates in the reaction zone, and the term "particle diameter of a catalyst" refers to the diameter when the catalyst has a spherical shape, refers to the major-axis length when the catalyst has a pellet shape, refers to the larger of the outer diameter of the circle and the height when the catalyst has a cylindrical or ring shape, refers to the outer diameter of the circle when the catalyst has a disk shape, refers to the dimension of the longest distance between two points on the periphery of a section formed by cutting the shape perpendicularly to the thickness direction when the catalyst has a platy shape, and refers to the axial-direction length when the catalyst has a rod shape.