The present invention provides a cell line which can be substituted for .beta. cells in human mature pancreatic islets and express insulin in a glucose-concentration dependent manner, and enables the easy obtainment of the number of cells which meets the demand. The present invention also provides a therapeutical cell preparation for treating diabetes. The cell lines of the present invention can be obtained by integrating both a nucleotide sequence encoding tamoxifen-induced Cre recombinase and a nucleotide sequence encoding insulin regulated by glucose-sensitive promoter into the chromosome in a human immortalized hepatic cell line FERM BP-7498 containing the TERT gene inserted in between a pair of LoxP sequences.