Proton decay from the 3 □ ω isoscalar giant dipole resonance (ISGDR) in Ni58 has been measured using the (α,α'p) reaction at a bombarding energy of 386MeV to investigate its decay properties. We have extracted the ISGDR strength under the coincidence condition between inelastically scattered α particles at forward angles and decay protons emitted at backward angles. Branching ratios for proton decay to low-lying states of Co57 have been determined, and the results compared with predictions of recent continuum-RPA calculations. The final-state spectra of protons decaying to the low-lying states in Co57 were analyzed for a more detailed understanding of the structure of the ISGDR. It is found that there are differences in the structure of the ISGDR as a function of excitation energy.