We propose an optimal approximation formula for analytic functions that are defined on a complex region containing the real interval $(-1,1)$ and possibly have algebraic singularities at the endpoints of the interval. As a space of such functions,we consider a Hardy space with the weight given by $w_{mu}(z) = (1-z^{2})^{mu/2}$ for $mu > 0$, and formulate the optimality of an approximation formula for the functions in the space. Then, we propose an optimal approximation formula for the space for any $mu > 0$ as opposed to existing results with the restriction $0 < mu < mu_{ast}$ for a certain constant $mu_{ast}$. We also provide the results of numerical experiments to show the performance of the proposed formula.
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