A temperature sensor includes a sensing member, a retention member configured to secure the sensing member, an optical fiber configured to irradiate the sensing member with light and guide the light reflected from the sensing member, and a cylindrical sleeve configured to accommodate the optical fiber. The retention member is a plate-shaped component and has a cut-out portion formed on at least one of a peripheral portion of a non-retention surface of the retention member opposite to a retention surface to which the sensing member is secured and a side surface of the retention member. The retention member is secured to a tip of the sleeve so that the non-retention surface is exposed to the outside, and the tip of the sleeve is engaged with the cut-out portion.