It has been known that the cosmic star formation rate density reaches its peak at z~1-3, while the problem that why this phenomenon occurs has not been solved yet. To address this puzzle, studies of molecular gas density evolution have been conducted by measuring CO line luminosities of known galaxies. However, the results might be biased due to optical/NIR pre-selections. To estimate molecular gas density without any bias, we conduct a blind search for line emitters and thus constrain CO line luminosity functions using ALMA 3mm data. The results will be discussed in this conference.