To provide a biomass gasifier, which may be a small-scale one, that produces a highly calorific clean fuel gas suited for a power generation gas engine in high yields from biomass resources.

In the biomass gasifier wherein steam is fed into crushed biomass to cause a gasification reaction in a gasification space, the gasification space is isolated from external heating by a partition, and the gasification reaction of the steam with the biomass is caused by an endothermic reaction without actively feeding oxygen into the gasification space. The partition has a gasification space in its inside, the inner side space for gasification by the external heating has a tubular structure, and the gasification space has a spouted bed structure in which the fed biomass powder is fluidized with the fed steam.