The nuclear structure calculations for strontium, zirconium, and molybdenum isotopes are performed in a large configuration space by employing a quadrupole-plus-pairing-interaction Hamiltonian. The possible factors responsible for the sudden onset of deformation at N = 60 in these nuclei are pointed out from the systematics of the occupancies of the single particle orbitals derived from the variational microscopic calculations. The energy spectra and the B(E2) values in the doubly even /sup 98/Sr, /sup 100,102/Zr, and /sup 104,106/Mo obtained from the variation-after-projection formalism are in good agreement with the corresponding experimental data.