  We give a relation between the existence of a Zariski decomposition and the
behavior of the restricted volume of a big divisor on a smooth (complex)
projective variety. Moreover, we give an analytic description of the restricted
volume in the line of Boucksom's work. It enables us to define the restricted
volume of a transcendental class on a compact K\"ahler manifold in natural way.
The relation can be extended to a transcendental class.
