Detection and sizing of a closed crack are difficult by the conventional ultrasonic testing, because ultrasonic waves propagate through the closed crack. Now nonlinear ultrasonic testing that uses subharmonics generated by effect of a closed crack is attracting attention. The objective of this study is to discuss a possible mechanism of the subharmonics in ultrasonic testing of a closed crack by introducing the idea of nonlinearity caused by gap-width-dependent property of interfacial stiffness, which successfully explained generation of superharmonics at a closed crack (Biwa, et al., 2005). We performed theoretical analysis of wave propagation in a continuum body based on a modeling including the above nonlinearity. Theoretical results predict generation of the subharmonics. They can appear if the input frequency close to twice the natural frequency. Conditions for the subharmonic resonance can also be found theoretically with respect to the input frequency and the input amplitude. We also carried out experiments of two aluminum specimens strongly pressed and verified our predicted results.