Cavitation in an aperture flow in a flat plate is studied using viscous potential flow. The maximum tension criterion for cavitation used here was proposed by Joseph [Phy. Rev. E, vol. 51, pp. 1649-1650, 1995; J. Fluid Mech., vol. 366, 367-378, 1998]: "Liquids at atmospheric pressure which cannot withstand tension will cavitate when and where tensile stresses due to motion exceed one atmosphere. A cavity will open in the direction of the maximum tensile stress which is 45 degrees from the plane of shearing in pure shear of a Newtonian fluid." The aperture flow is expressed using a complex potential and the stress is calculated using viscous potential flow. We find that the viscous stress is huge near the tips of the aperture, thus cavitation could be induced.