We propose consistency principle between different levels, to understand a biological system. Three topics are discussed. First, as a result of consistency between molecule replication and cell reproduction, universal statistical laws on chemical abundances over cells are derived as also confirmed experimentally. They include power-law distribution of gene expressions,log-normal distribution of chemical abundances over cells, and embedding of the power law into the network connectivity. Second, as a result of consistency between gene and phenotype,a general relationship between phenotype fluctuations by genetic variation and isogenic phenotypic fluctuation by developmental noise is derived. Third, we touch upon the chaos mechanism for stem cell differentiation with autonomous regulation, as a result of consistency of cell reproduction and growth of cell ensemble.