To provide an overcharge prevention device for liquefied gas, capable of smoothly emitting vaporized gas at a sufficient speed without providing a passage for emitting the vaporized gas separately nor applying load to a loose edge part of a diaphragm.

This overcharge prevention device for liquefied gas 1 comprises of a body 10 having an inlet port 11, an outlet port 16, an annular inflow chamber 13, a tubular valve seat wall 15, a valve seat 14, and a diaphragm chamber 17, the diaphragm 12 having a fixed edge part 22, the loose edge part 23, and a pore 20 and disposed at a position so as to separate the annular inflow chamber 13 from the diaphragm chamber 17 in the body 10, and a pilot valve 19. A reverse-flow prevention means (a partitioning plate 25 with a communicating hole 24, a small sphere 28, a fitting hole 29, and a spring 30) is provided for preventing reverse-flow of the vaporized gas in the diaphragm chamber 17 at the time of emitting the vaporized gas.