In this paper, we investigate the regularized mean curvature flow starting from an invariant hypersurface in a Hilbert space equipped with an isometric and almost free action of a Hilbert Lie group whose orbits are regularized minimal. We prove that, if the invariant hypersurface satisfies a certain kind of horizontally convexity condition and its image by the orbit map of the Hilbert Lie group action is included by the geodesic ball of some radius, then it collapses to an orbit of the Hilbert Lie group action along the regularized mean curvature flow. As an application of this result to the gauge theory, we derive a result for the behaviour of the holonomy elements (along a fixed loop) of connections belonging to some based gauge-invariant hypersurface in the space of connections of the principal bundle having a compact Lie group as the structure group over a compact Riemannian manifold along the regularized mean curvature flow starting from the hypersurface. almost all relevant cases, these regularized traces coincide. In this paper, we adopt the regularized trace defined in [HLO]. Let M be a proper Fredholm submanifold in V immersed by f . If, for each normal vector ξ of f , the regularized trace Tr r A ξ of the shape operator A ξ of f and the trace Tr A 2 ξ of A 2 ξ exist, then M (or f ) is said to be regularizable. See Section 3 about the definition of the regularized trace Tr r A ξ . Let M be a Hilbert manifold and f t (0 ≤ t < T ) be a C ∞ -family of regularizable immersions of codimension one of M into V which admit a unit normal vector field ξ t . The regularized mean curvature vector H t is defined by. We call f t 's (0 ≤ t < T ) the regularized mean curvature flow if the following evolution equation holds:R. S. Hamilton ([Ha]) proved the existenceness and the uniqueness (in short time) of solutions of a weakly parabolic equation for sections of a finite dimensional vector bundle. The evolution equation (1.1) is regarded as the evolution equation for sections of the infinite dimensional trivial vector bundle M × V over M . Also, M is not compact. Hence we cannot apply the Hamilton's result to this evolution equation (1. 1). Also, we must impose certain kind of infinite dimensional invariantness for f because M is not compact. Thus, we cannot show the existenceness and the uniqueness (in short time) of solutions of (1.1) in general. However we ([Koi]) showed the existenceness and the uniqueness (in short time) of solutions of (1.1) in the following special case. We consider a isometric almost free action of a Hilbert Lie group G on a Hilbert space V whose orbits are regularized minimal, that is, they are regularizable submanifold and their regularized mean curvature vectors vanish, where "almost free" means that the isotropy group of the action at each point is finite. Denote by N the orbit space V /G, which is an orbifold. Give N the Riemannian orbimetric g N such that the orbit map φ : V → N is a Riemannian orbisubmersion. Let M (⊂ V ) be a G-invariant submanifold in V . Assume that M := φ(M ) is compa...