It is aimed to improve reaction efficiency by increasing a contact area of first and second reactants per unit volume without reducing dimensions of an inlet path for the first reactant and an inlet path for the second reactant in a layer-thickness direction.In a reactor, a channel includes a first inlet path having the first reactant introduced thereinto, a second inlet path arranged while being separated from the first inlet path and having the second reactant introduced thereinto, a junction channel for causing the first reactant flowing via the first inlet path and the second reactant flowing via the second inlet path to join in the form of the laminar flows separated from each other, and a reaction channel connected with a downstream side of the junction channel for permitting the laminar flow of the first reactant and that of the second reactant held in contact with each other and reacting the two reactants at a contact interface thereof. A dimension of the reaction channel in the layer-thickness direction perpendicular to the contact interface is set to be smaller than the sum of a dimension of the first inlet path in the layer-thickness direction and a dimension of the second inlet path in the layer-thickness direction.