The electrochemical quartz crystal microbalance (EQCM) technique was successfully used to investigate the lithium insertion/extraction reaction in LiMn2O4 spinel. A uniform and dense film of LiMn2O4 was prepared by electrostatic spray deposition (ESD) onto an Au-coated quartz plate, which was used as an electrode for the EQCM experiments. The ESD method enabled us to perform the low-temperature spinel synthesis at 400 °C to avoid the thermal damage of the quartz. The electrochemical properties of the LiMn2O4 film in a 1 M LiClO4 carbonate solution were studied at 50 °C. The data obtained for the lithium insertion/extraction were discussed for the first time in connection with the mass change of the LiMn2O4 specimen.