The α-particle condensed state in 16O is investigated by the use of 4α OCM (Orthogonality Condition Model). Low-lying spectrum in experiment is well reproduced up to around the 4α threshold. Furthermore, the excited state obtained around the 4α threshold as the state is shown to have a dilute density structure and give strong enhancement of the occupation of the S-state c.o.m. orbital of the α-particles. This is strong evidence that the state is regarded as the 4α condensed state. It also has a large component of configuration, which is another reliable evidence of the state to be of 4α condensate nature.