The most degenerate unitary principal series representations π iλ,δ (λ ∈ R, δ ∈ Z/2Z) of G = GL(N, R) attain the minimum of the Gelfand-Kirillov dimension among all irreducible unitary representations of G. This article gives an explicit formula of the irreducible decomposition of the restriction π iλ,δ | H (branching law) with respect to all symmetric pairs (G, H ). For N = 2n with n 2, the restriction π iλ,δ | H remains irreducible for H = Sp(n, R) if λ = 0 and splits into two irreducible representations if λ = 0. The branching law of the restriction π iλ,δ | H is purely discrete for H = GL(n, C), consists only of continuous spectrum for H = GL(p, R) × GL(q, R) (p + q = N), and contains both discrete and continuous spectra for H = O(p, q) (p > q 1). Our emphasis is laid on geometric analysis, which arises from the restriction of 'small representations' to various subgroups.