An object of the present invention is to provide a method capable of inexpensively and conveniently preparing cells having pluripotency and a very low risk of tumorigenic transformation. The cells can be prepared by suspension-culturing mammalian mesenchymal stem cells such as human mesenchymal stem cells from bone marrow (hMSC-BM) and human adipose tissue-derived mesenchymal stem cells (hAT-MSC) (also referred to as “human adipose-derived stem cells [hADSC]”), 7 types of human adherent mature cells (human hepatocyte cells [hHEP cells], human umbilical vein endothelial cells [HUVEC cells], human dermal lymphatic microvascular endothelial cells [HMVEC cells], human epidermal keratinocyte cells [NHEK cells], human bronchial epithelial cells [NHBE cells], human melanocyte cells [NHEM cells], and human smooth muscle cells [UASMC cells]), and 3 types of human adherent precursor cells (human dermal fibroblast cells [NHDF cells], human skeletal muscle myoblast cells [HSMM cells], and human osteoblast cells [NHOst cells]) to form cell masses (spheroids).