Chloride channels regulate cell volume by an efflux of chloride ions in response to osmotic stresses. These have been shown to play a role in cancer invasion. However, their function in cancer metastasis remains unclear. As the internal environment of the human body is rarely exposed to osmotic stress, we presumed that Cl- efflux in cancer cells is induced by mechanical stress caused by their crowded environment and invasion of their narrow interstitial spaces. In this study, we recruited atomic force microscopy to apply mechanical stress to mouse or human breast cancer cells with varying degrees of malignancy and examined their Cl- efflux by N-ethoxycarbonylmethyl-6-methoxyquinolinium bromide (MQAE), which is quenched via collision with Cl- ions. We found that intracellular MQAE fluorescence intensity increased immediately after cell compression, demonstrating induction of Cl- efflux by mechanical force. Furthermore, Cl- efflux ability showed correlation with the cancer metastatic potential. These results suggested that mechanical stress induced Cl- efflux may serve as a potential reporter for estimating the invasion ability of cancer cells.