In order to improve sealing properties between a downcomer 10 and a high-speed layer 20 and in order to efficiently take out produced gas obtained through gasification of, for example, a solid reactant, a syphon 30 for sealing between a downcomer 10 and a high-speed layer 20 through temporary storage of particles moving from the downcomer 10 to the high-speed layer 20 comprises a reactor portion 70 for causing the solid reactant to conduct chemical reaction through action of the particles, a downcomer seal portion 40 in communication, at upper and lower ends thereof, with the downcomer 10 and a lower portion the reactor portion 70, respectively, a particle outlet seal portion 50 provided in a spaced apart relationship from the downcomer seal portion 40 and in communication, at upper and lower ends thereof, with the high-speed layer 20 and the lower portion of the reactor portion 70, respectively, and a freeboard portion 76 formed above the reactor portion 70.