This paper describes the influence of a high-optical power light launched into optical fibers in an mechanically transferable (MT) connector. The features of the MT connector are the use of index matching material to suppress Fresnel reflection and the accommodation of several optical fibers to allow the connection of optical fiber ribbons. First, we experimentally investigated the relationship between connection loss, launched light power, temperature increase, and optical loss increase using an MT connector sample without contamination. Based on the results, we clarified the relationship between the allowed connection loss and the launched optical power in the optical fibers needed to maintain MT connector reliability. In addition, we examined the influence of the contamination of the connector endface and confirmed that it is necessary to clean the endface appropriately in an optical transmission system that uses high-optical power light to prevent any fatal damage to the MT connector.