  We investigate the soft dipole resonance in the proton-rich nucleus
$^8_6$C$_2$, which is a collective dipole oscillation of four valence protons
against the $\alpha$ core,and discuss the isospin symmetry with the mirror
nucleus $^8_2$He$_6$. We use the $\alpha+N+N+N+N$ five-body cluster model and
many-body resonances are obtained using the complex-scaling method. The $1^-$
resonance of $^8_6$C$_2$ is confirmed at the excitation energy of 13 MeV with a
large decay width of 24 MeV, and its structure is similar to the soft dipole
resonance in $^8_2$He$_6$, such as the configuration mixing and the spatial
properties. These results indicate a good isospin symmetry in the soft dipole
resonances of two nuclei with a common collective excitation of multiproton and
multineutron, while the ground states of two nuclei show different properties
due to the Coulomb repulsion of valence protons in $^8_6$C$_2$, leading to the
symmetry breaking. In conclusion, the appearance of the isospin symmetry
differs depending on the states of $^8_6$C$_2$ and $^8_2$He$_6$.
