The present invention relates to a method for monitoring migration, invasion, and metastasis of cancer cells by observing the shape of cancer cells cultured in a three-dimensional environment and measuring the activity, expression, and changes in expression sites of proteins associated with invadopodia formation and metastasis, and the degradation of an extracellular matrix; and to a method for screening a cancer metastasis inhibitor. More specifically, it was verified that the reduction in c-Jun phosphorylation induced the increase in snail1 and the decrease in cortactin expression in the cells cultured in a three-dimensional environment and the expression regulation relations between the proteins were identical to those in breast cancer tissues obtained from patients. In addition, it was verified that, when breast cancer cells in a three-dimensional collagen gel environment were treated with a JNK inhibitor, the shape of the cells became longer; the contact region of the cells and the extracellular matrix became flattened and thinner; the migration of cancer cells was decreased; and the changes in protein expression was observed, such as the increase in TGFβ1 expression, the increases in smad2 and smad3 expression and activity, the increase in snail1 expression, the decrease in cortactin expression, and the resulting decrease in invadopodia formation. In addition, in a three-dimensional collagen gel environment, MT1-MMP besides the cortactin can be used as a marker of invadopodia, and it was verified that the inhibition of JNK led to the decrease in cortactin expression and the increase in snail1 expression, badly influenced the site and role of MT1-MMP to inhibit the formation of invadopodia, and inhibited the degrading activity of a collagen gel substrate. Thus, the present invention can be used as a method for monitoring migration, invasion, metastasis, and the degree of metastasis of cancer cells and a method for screening a cancer metastasis inhibitor, and will be useful as one of screening methods capable of creating low-cost, high-efficient added value at the time of pre-clinical tests required for drug development.