Let k ∞ be the cyclotomic Z p -extension of an algebraic number field k. We denote by S a finite set of prime numbers which does not contain p, and S(k ∞ ) the set of primes of k ∞ lying above S. In the present paper, we will study the structure of the Galois group X S (k ∞ ) of the maximal pro-p extension unramified outside S(k ∞ ) over k ∞ . We mainly consider the question whether X S (k ∞ ) is a non-abelian free pro-p group or not. In the former part, we treat the case when k is an imaginary quadratic field and S = ∅ (here p is an odd prime number which does not split in k). In the latter part, we treat the case when k is a totally real field and S = ∅.