© 2017 American Physical Society. We present a microscopic calculation of α-cluster formation in heavy nuclei by using the quartetting wave function approach. The interaction of the quartet with the core nucleus is taken in local density approximation. The α-cluster formation is found to be particularly sensitive to the interplay of the mean field felt by the α-cluster and the Pauli blocking as a consequence of antisymmetrization. The striking feature of α-cluster formation probability across the major shell closures of 82 protons and 126 neutrons is reproduced. The shell (or subshell) effects on the α-cluster formation in superheavy nuclei are also analyzed.