The fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), is an economically important polyphagous pest, which has recently expanded its distribution from the Americas to over 70 countries. Here, we established an efficient rearing method for S. frugiperda under crowded conditions. First, we clarified the larval density at which there is no decrease in survival rate due to cannibalism. Spodoptera frugiperda larvae were maintained on an artificial diet till pupation under five density conditions: 10, 30, 50, 70, and 100 individuals per container (5.4L, 26.8 cm × 20.7 cm × 9.8 cm). Survival rates at adult emergence were approximately 70% at density conditions of less than 50, and those in the container with paper towels as refuge were higher than those in the container without paper towels. In the second experiment, two individuals at different developmental stages from 5th instar to pupa were released in a container with or without food, and with or without paper towels, to clarify the developmental stages at which cannibalism occurs. Even when food was present, the individuals without access to refuge during the prepupa were attacked by the 6th instar larva. Our results suggested that conditions which include refuge and moderate density facilitate the rearing of S. frugiperda larvae under crowded conditions.