The properties of phase time taken for particles to pass through a quantum potential well are investigated. It is found in a l-dimensional quantum-mechanical problem that the phase time is negative when the incident energy and the thickness of potential well satisfy certain conditions. Similar results are also found in a 2-dimensional fully relativistic optical analog. It is shown that the expression of the lateral shift of transmitted optical waves is similar to that of the phase time in the 1-dimensional quantum-mechanical problem. The phase time in the 2-dimensional optical problem is also shown to be negative under certain conditions.