  The notion of trajectory of an individual particle is strictly inhibited in
quantum mechanics because of the uncertainty principle. Nonetheless, the weak
value, which has been proposed as a novel and measurable quantity definable to
any quantum observable, can offer a possible description of trajectory on
account of its statistical nature of the value. In this paper, we explore the
physical significance provided by this weak trajectory by considering various
situations where interference takes place simultaneously with the observation
of particles, that is, in prototypical quantum situations for which no
classical treatment is available. These include the double slit experiment and
Lloyd's mirror, where in the former case it is argued that the real part of the
weak trajectory describes an average over the possible classical trajectories
involved in the process, and that the imaginary part is related to the
variation of interference. It is shown that this average interpretation of the
weak trajectory holds universally under the complex probability defined from
the given transition process. These features remain essentially unaltered in
the case of Lloyd's mirror where interference occurs with a single slit.
