The purpose of this study is to develop a general method of facility location by Genetic Algorithm based on Co-Evolution. To this end, this article proposes a new method which forms an individual by the following two chromosomes, that is, 'location chromosome' and 'allocation chromosome'. These two chromosomes co-evolve in an individual. The proposed method is applied to a location-allocation problem of day facilities for the elderly in Karasuyama district of Setagaya-ku, Tokyo. The results are as follows: 1) this method shows an excellent ability to search a solution; 2) the solutions are obtained through a series of alternate trial-and-error by two chromosomes; 3) this method searches a solution which balances various factors of facility location, which gives practicality to the proposed method.