An internal combustion engine with an engine body and a retaining structure for a temperature detection device, comprising: a resin cooling medium circulation part configured to form a portion of a circulation path for circulating a cooling medium for cooling an engine body; and an attachment part (12, 21) configured to retain the temperature detection device for sensing the temperature of said cooling medium flowing through said circulation path, so that a temperature sensing part of said temperature detection device protrudes into said circulation path, the attachment part being a material capable of transmitting heat from the engine body to said temperature detection device, and being attached to said resin cooling medium circulation part such that a portion of the attachment part is configured to contact said engine body; said attachment part (12, 21) has an abutting part configured to abut said engine body in planar manner; said attachment part (12, 21) has: a retaining part configured to retain said temperature detection device, and a transmitting member as said abutting part disposed in a joining portion between said resin cooling medium circulation part and said engine body and configured to transmit heat of said engine body to said attachment part; characterized in that said abutting part is fixed to said engine body so as to contact two faces of said engine body in a planar manner, the two faces opposing each other and forming a recess formed on a side of said engine body. a resin cooling medium circulation part configured to form a portion of a circulation path for circulating a cooling medium for cooling an engine body; and an attachment part (12, 21) configured to retain the temperature detection device for sensing the temperature of said cooling medium flowing through said circulation path, so that a temperature sensing part of said temperature detection device protrudes into said circulation path, the attachment part being a material capable of transmitting heat from the engine body to said temperature detection device, and being attached to said resin cooling medium circulation part such that a portion of the attachment part is configured to contact said engine body; said attachment part (12, 21) has an abutting part configured to abut said engine body in planar manner; said attachment part (12, 21) has: a retaining part configured to retain said temperature detection device, and a transmitting member as said abutting part disposed in a joining portion between said resin cooling medium circulation part and said engine body and configured to transmit heat of said engine body to said attachment part; characterized in that a retaining part configured to retain said temperature detection device, and a transmitting member as said abutting part disposed in a joining portion between said resin cooling medium circulation part and said engine body and configured to transmit heat of said engine body to said attachment part; characterized in that said abutting part is fixed to said engine body so as to contact two faces of said engine body in a planar manner, the two faces opposing each other and forming a recess formed on a side of said engine body.