Gauss established a theory on the functional equation (Gauss' functional equation)
f(a+b2,ab)=f(a,b) (a,b>0),
where f:R+×R+→R is an unknown function of the above equation.In this paper we treat the functional equation
f(a+b2,2aba+b)=f(a,b) (a,b>0)
where f:R+×R+→R is an unknown function of this equation.The purpose of this paper is to prove new results on this functional equation by following
the theory of Gauss' functional equation.