A seat occupancy detection device comprising: a membrane switch that forms a seat occupancy detection section on a seat occupancy surface of a seat cushion; and a seat occupancy determination device that determines a seat occupancy state of an occupant with respect to the seat cushion, based on an ON/OFF state of the membrane switch, wherein a plurality of detection regions (α1, α2, β1, β2) are set on the seat occupancy detection section by dividing the seat occupancy detection section in a frontward-rearward direction and in a width direction of a seat, and characterized in that the membrane switch is configured to enter an ON state in a case where pressure-sensitive switch units (SW, SW1, SW2, SW3, SW4) arranged in two detection regions, respectively, which are adjacent in the frontward-rearward direction of the seat, simultaneously enter a conduction state, wherein the membrane switch includes a first film and a second film which are laminated with an intermediate film interposed therebetween, a first circuit pattern and a second circuit pattern which are formed on the first film, and a third circuit pattern which is formed on the second film, wherein the respective pressure-sensitive switch units have a configuration in which respective contact portions (X, X1, X2) of the first circuit pattern and the second circuit pattern face respective contact portions (X, X3) of the third circuit pattern via a communication portion provided in the intermediate film, wherein the pressure-sensitive switch unit formed by the contact portion of the first circuit pattern is disposed on one of the two detection regions adjacent in the frontward-rearward direction of the seat, and the pressure-sensitive switch unit formed by the contact portion of the second circuit pattern is disposed on the other detection region, and wherein the membrane switch is configured to be with a first connection terminal provided on one end of the first circuit pattern and a second connection terminal provided on one end of the second circuit pattern as connection portions. a membrane switch that forms a seat occupancy detection section on a seat occupancy surface of a seat cushion; and a seat occupancy determination device that determines a seat occupancy state of an occupant with respect to the seat cushion, based on an ON/OFF state of the membrane switch, wherein a plurality of detection regions (α1, α2, β1, β2) are set on the seat occupancy detection section by dividing the seat occupancy detection section in a frontward-rearward direction and in a width direction of a seat, and characterized in that the membrane switch is configured to enter an ON state in a case where pressure-sensitive switch units (SW, SW1, SW2, SW3, SW4) arranged in two detection regions, respectively, which are adjacent in the frontward-rearward direction of the seat, simultaneously enter a conduction state, wherein the membrane switch includes a first film and a second film which are laminated with an intermediate film interposed therebetween, a first circuit pattern and a second circuit pattern which are formed on the first film, and a third circuit pattern which is formed on the second film, a first film and a second film which are laminated with an intermediate film interposed therebetween, a first circuit pattern and a second circuit pattern which are formed on the first film, and a third circuit pattern which is formed on the second film, wherein the respective pressure-sensitive switch units have a configuration in which respective contact portions (X, X1, X2) of the first circuit pattern and the second circuit pattern face respective contact portions (X, X3) of the third circuit pattern via a communication portion provided in the intermediate film, wherein the pressure-sensitive switch unit formed by the contact portion of the first circuit pattern is disposed on one of the two detection regions adjacent in the frontward-rearward direction of the seat, and the pressure-sensitive switch unit formed by the contact portion of the second circuit pattern is disposed on the other detection region, and wherein the membrane switch is configured to be with a first connection terminal provided on one end of the first circuit pattern and a second connection terminal provided on one end of the second circuit pattern as connection portions.