We performed in vitro assays to visualize the effects of pressure on the filamentous structure of microtubules. Taxol-stabilized microtubules were tethered to kinesin motors on the observation window of a high-pressure chamber. When pressure was applied to the sample solution, all of the microtubules started to shorten from both ends. The length changes were constant over time, irrespective of the microtubule polarity. The shortening rate of microtubules increased exponentially with pressure, and the activation volume was -100 mL/mol, consistent with in vivo studies. These results show that application of pressure works directly to weaken the intermolecular interactions between tubulin molecules.