Division of labor performed by social insects, such as ants and bees, is one of the most advanced functions that they have evolved. Although individual insects have the same inheritance, they are divided into certain groups in which they play specialized roles. For effective performance, the groups exhibit proportional regulation of population, which implies regulating their population to maintain their proportion; further the proportion of each group is regulated against external disturbances without aid from a special individual. In a multi-robot system, the division of labor is one of the most attractive and useful propositions. This paper addresses the proportion regulation of population in a homogenous multi-robot system, which is inspired by the task allocation practiced by social insects. We propose an extended probability model for proportion regulation, and confirm its performance by numerical simulation. We also experimentally validate its performance using a simple multi-robot system.