Abstract. Roughly speaking, the regular subspace of a Dirichlet form is also a regular Dirichlet form on the same state space. It inherits the same form of original Dirichlet form but possesses a smaller domain. What we are concerned in this paper are the regular subspaces of associated Dirichlet forms of skew product diffusions. A skew product diffusion X is a symmetric Markov process on the product state space E 1 × E 2 and expressed aswhere X i is a symmetric diffusion on E i for i = 1, 2, and A is a positive continuous additive functional of X 1 . One of our main results indicates that any skew product type regular subspace of X, saycan be characterized as follows: the associated smooth measure ofà is equal to that of A, and Y i corresponds to a regular subspace of X i for i = 1, 2. Furthermore, we shall make some discussions on rotationally invariant diffusions on R d \ {0}, which are special skew product diffusions on (0, ∞) × S d−1 . Our main purpose is to extend a regular subspace of rotationally invariant diffusion on R d \ {0} to a new regular Dirichlet form on R d . More precisely, fix a regular Dirichlet form (E, F ) on the state space R d . Its part Dirichlet form on R d \ {0} is denoted by (E 0 , F 0 ). Let (Ẽ 0 ,F 0 ) be a regular subspace of (E 0 , F 0 ). We want to find a regular subspace (Ẽ,F ) of (E, F ) such that the part Dirichlet form of (Ẽ,F ) on R d \ {0} is exactly (Ẽ 0 ,F 0 ). If (Ẽ ,F ) exists, we call it a regular extension of (Ẽ 0 ,F 0 ). We shall prove that under a mild assumption, any rotationally invariant type regular subspace of (E 0 , F 0 ) has a unique regular extension.