It is an object of the present invention to provide a raw material powder for stably obtaining a dense sinter that is prevented from cracking, and a method for manufacturing this powder, and a method for manufacturing a lanthanum-based oxide ion conductor in which this raw material powder is used.The raw material powder manufacturing method of the present invention is a method for manufacturing a raw material powder for forming an oxide ion conductor composed of a multi-component metal oxide including lanthanum or lanthanide, wherein a mixed powder blended such that all of the elements constituting said multi-component metal oxide are included is prefired, after which this prefired powder is exposed to water or moist gas so as to expand at least some of the particles in said powder. Alternatively, two types of mixed powder with different components are prefired separately, after which the prefired powders are blended in a specific ratio.