Most biological materials utilized in odor biosensors require an aqueous environment for retaining their function. However, the buffer medium surrounding the sensing materials can be a barrier for hydrophobic odorant molecules to penetrate. There are two methods to solve this problem. The first is the addition of molecular transporters, e.g., odorant binding proteins. The second is to cover the sensing materials with a sufficiently thin liquid layer. Considering that a thin liquid layer is easy to realize and inexpensive, we focused on this approach for direct gas-phase odor sensing. Here, we present diverse methods—no additional control, open-loop control, and closed-loop control—for maintaining a thin liquid layer in the odor biosensor. We recorded the impedance curve under liquid natural evaporation and proposed an impedance equivalent circuit model. We investigated the reason for the early stopping of the biosensor response. Moreover, we demonstrated the performance of a suitable closed-loop control method in fine tuning the liquid layer thickness. The methods reported in this paper would contribute to enabling the practical application of odor biosensors.