The endoplasmic reticulum (ER) is an organelle in which proteins are modified. When unfolded proteins accumulate in the ER under various stresses, ER stress (ERS) pathways, including the induction of chaperones, are activated to protect the cell. However, when ERS is excessive, the cell undergoes apoptosis. This study investigated ERS in multiple myeloma cells (MMCs) because they contain a well-developed ER due to M-protein production. The myeloma cell line 12-PE underwent apoptosis via caspase-3 after treatment with thapsigargin (thap), an ERS inducer, while another cell line, U266, did not. To understand the mechanism regulating this heterogeneity, the induction of chaperones by thap was analysed. Chaperones were up-regulated in U266 cells but down-regulated in 12-PE cells, suggesting that chaperones contribute to cell survival under ERS. Analysis of XBP-1, a transcriptional inducer of chaperones, in freshly isolated MMCs from 22 myeloma cases revealed 10 cases with active XBP-1, who also showed significantly poorer survival (p < 0.05), suggesting that chaperone expression protects MMCs from apoptosis, thereby allowing tumor cell expansion. These results suggest that MMCs are subjected to ERS under certain circumstances and that chaperones are induced to protect the cells against such ERS. Inhibition of chaperones could be a new target for myeloma therapy.