The present invention relates to a gene regulating flowering time and a method for regulating flowering time in plants using the same. More particularly, the present invention relates to a COG gene having nucleotide sequence represented by SEQ ID No: 1 which is isolated from Arabidopsis thaliana, and a method for delaying flowering time of plants by overexpressing the gene, or for inducing early flowering by repressing an expression of the gene. The COG gene and the COG protein expressed therefrom according to the present invention are useful for improvement of flowering-associated character of plants, and for identification of flowering-associated genes or proteins in other plants, etc.