We present an in-situ atomic force microscopy (AFM) study of spherical lipid membranes (also known as liposomes) on a substrate under liquid. The in-situ AFM observation has been carried out using a commercially available AFM instrument equipped with a fluid cell in the contact mode. An immunochemical technique was used to prepare the sample. The AFM imaging of the liposomes showed their balloon-like structures with apparent diameters of approximately 200-300 nm. These diameters are essentially in agreement with a mean size of 180 nm deduced from a previous dynamic light-scattering experiment, taking account of the well-known broadening phenomenon of lateral sizes in AFM images due to the tip-sample convolution. It is shown that the quality (contrast and/or reproducibility) of the AFM images depends both on the types of cantilever tips used and on the load forces at which the AFM tips were scanned over the sample. Based upon this, it is concluded that there is a certain critical contact pressure of a probe tip over the sample, which determines the quality of the sample image taken under water.