The present invention relates to a pharmaceutical composition for preventing or treating diabetes or a complication of diabetes, which includes a TENC1 (tensin like C1 domain-containing phosphatase) expression or activity suppressor, and, more specifically, relates to a pharmaceutical composition for preventing or treating diabetes or a complication of diabetes, which either suppresses the degradation of IRS-1 (insulin receptor substrate-1) or suppresses the phosphorylation of IRS-1 due to the PTPase activity of TENC1. The pharmaceutical composition according to the present invention, which is for preventing or treating diabetes or a complication of diabetes and comprises the TENC1 expression or activity suppressor as an active ingredient, can be expected to be widely usable in preventing and treating diabetes or a complication of diabetes since the pharmaceutical composition can effectively prevent the muscular dystrophy and reduction in sugar adsorption that occur due to reduction in IRS-1 by suppressing degradation of IRS-1 caused by TENC1.