We carry out three-body-model calculations for a proton-rich Borromean nucleus by assuming a structure. To this end, we use a density-dependent contact interaction between the valence protons, explicitly treating also the Coulomb interaction. We find that the two-particle density distribution for is similar to that for , which has two valence neutrons outside the core. That is, the two protons take a spatially compact configuration, while the Coulomb repulsion plays a minor role. This indicates that there is a strong diproton correlation in the ground state of the nucleus. We also show that the Coulomb interaction reduces the expectation value of the proton-proton interaction by about .