  We study the resonance spectroscopy of the proton-rich nucleus 8C in the
alpha+p+p+p+p cluster model. Many-body resonances are treated on the correct
boundary condition as the Gamow states using the complex scaling method. We
obtain the ground state of 8C as a five-body resonance for the first time,
which has dominantly the sub-closed (p_{3/2})^4 configuration and agrees with
the recent experiment for energy and decay width. We predict the second 0+
state with the excitation energy of 5.6 MeV, which corresponds to the $2p2h$
state from the ground state. We evaluate the occupation numbers of four
valence-protons in the 8C states and also the J^\pi distribution of proton-pair
numbers of the two 0+ states of 8C. The ground state involves a large amount of
the 2+ proton-pair fraction, while the excited 0+_2 state almost consists of
two of the 0+ proton pairs, which can be understood from the
(p_{3/2})^2(p_{1/2})^2 configuration. We also discuss the mirror symmetry
between 8C and 8He with an alpha+four nucleon picture. It is found that the 0+
states retain the mirror symmetry well for the configuration properties of two
nuclei.
