The present invention provides a-hydroxy benzeneacetic acid derivatives of the formula as defined in the specification which is a precursor indispensable for synthesis of compounds having two 5-membered lactone rings fused to central cyclohexa-1,4-diene nucleus, and a process of easily preparing the same. According to the preparation process of the present invention, the a-hydroxy benzeneacetic acid derivative can be readily prepared at high purity and yield without using toxic materials or producing toxic by-products. Some novel compounds, synthesized by using such alpha a-hydroxy benzeneacetic acid derivative, have excellent fastness properties, dye fixing rate and leveling property to general synthetic fiber materials such as polyester fibers and their blends with other fibers, especially to micro fibers, and also can be used as a coloring agent for plastic resins, color tonors, color filters, etc.