Atomic force microscopy (AFM) is a novel tool that can be used not only for topographical imaging but also for force measurements between substrates and particle probes (e.g., inorganic colloids or biocolloids). In this study, the focus is on the adhesion force between spores, such as thuringiensis spores, and flat surfaces. The morphology of Bacillus thuringiensis spores was obtained by AFM as well as by high-resolution scanning transmission electron microscopy. AFM was used for the measurement of the adhesion force between a flat surface and a spore probe. The adhesion force was measured with various substrates and at different humidity levels. The shape and dimensions of the spores depend on the relative humidity in an atmospheric environment. In a low humidity environment, the size of the spore becomes smaller and the surface has more roughness than in a high humidity environment. Such behavior could be explained by the protective structure of the outer membrane of spores. The coat, a layer of substructure on the spore surface, modifies its form to preserve DNA in the core. The adhesion force in atmospheric conditions was assumed to mainly consist of van der Waals attraction and capillary force. The van der Waals and capillary forces for colloidal particles are generally calculated with consideration of the shape, size, and roughness of particles. The combination of changes in size and morphology with each component of the adhesion force may explain the adhesion force behavior between flat surfaces and spores at different humidity levels.