A thermochromic writing instrument capable of causing curiosity and interest of a user by configuring an eraser to be easily separated from the main body of the thermochromic writing instrument and to be shaped into various character dolls. A rotation member is inserted into a pen body and a hollow upper cover is detachably coupled to an outside of the upper portion of the pen body, a first eraser is connected to the rotation member by a connection member, a connection portion of an operation member is disposed in a groove formed at one side of the side wall of the pen body, the first body portion of the operation member is inserted into the pen body and a clip portion of the operation member is exposed to the outside of the pen body.