We study the global existence of solutions to the Cauchy problem for the wave equation with time-dependent damping and a power nonlinearity in the overdamping case:whose case is called overdamping. N (u) denotes the p-th order power nonlinearities. It is well known that the problem is locally well-posed in the energy spaceIt is known that when N (u) := ±|u| p , small data blow-up in L 1 -framework occurs in the case b(t) −1 / ∈ L 1 (0, ∞) and 1 < p < pc(< p 1 ), where pc is a critical exponent, i.e. threshold exponent dividing the small data global existence and the small data blow-up. The main purpose in the present paper is to prove the global well-posedness to the problem for small data (u 0 , u 1 ) ∈ H 1 (R d ) × L 2 (R d ) in the whole energy-subcritical case, i.e. 1 ≤ p < p 1 . This result implies that the small data blow-up does not occur in the overdamping case, different from the other case b(t) −1 / ∈ L 1 (0, ∞), i.e. the effective or non-effective damping.Here, d ∈ N, T > 0, u = u(t, x) is a real-valued unknown function of (t, x), b = b(t) is a positive C 1 -function of t ∈ [0, ∞) satisfying b(t) −1 ∈ L 1 (0, ∞) i.e. ∞ 0 b(t) −1 dt < ∞, (1.2)