Summary: Corresponding to the irreducible 0-1 matrix \((a_{ij})_{n\times n}\), take similitude contraction mappings \(\varphi_{ij}\) for each \(a_{ij}= 1\) in \(\mathbb{R}^d\) with ratio \(0< r_{ij}< 1\). There are unique nonempty compact sets \(F_1,\dots, F_n\) satisfying for each \(1\leq i\leq n\), \(F_i= \bigcup^n_{\substack{ j=1\\ a_{ij}=1}} \varphi_{ij}(F_j)\). We prove that the open set condition holds if and only if \(F_i\) is an \(s\)-set for some \(1\leq i\leq n\), where \(s\) is such that the spectral radius of the matrix \((r^s_{ij})_{n\times n}\) is \(1\).