  The isotopic ratio of nitrogen presents a wide range of values in the Solar
System and in star forming system whose origin is still unclear. Chemical
reactions in the gas phase are one of the possible processes that could modify
the $^{14}$N/$^{15}$N ratio. We aim at investigating if and how the passage of
a shock wave in the interstellar medium, can affect the relative fraction of
nitrogen isotopes. The ideal place for such a study is the L1157 outflow, where
several shocked clumps are present. We present the first measurement of the
$^{14}$N/$^{15}$N ratio in the two shocked clumps, B1 and B0, of the
protostellar outflow L1157, derived from the interferomteric maps of the
H$^{13}$CN(1-0) and the HC$^{15}$N(1-0) lines. In B1, we find that the
H$^{13}$CN(1-0) and HC$^{15}$N(1-0) emission traces the front of the clump,
with averaged column density of $N$(H$^{13}$CN) $\sim$ 7$\times$10$^{12}$
cm$^{-2}$ and $N$(HC$^{15}$N) $\sim$ 2$\times$10$^{12}$ cm$^{-2}$. In this
region the ratio H$^{13}$CN(1-0)/HC$^{15}$N(1-0) is quite uniform with an
average value of $\sim$ 5$\pm$1. The same average value is also measured in the
smaller clump B0e. Assuming the standard $^{12}$C/$^{13}$C = 68, we obtain
$^{14}$N/$^{15}$N = 340$\pm$70, similar to those usually found in prestellar
cores and protostars. We analysed the prediction of a chemical shock model for
several shock conditions and we found that the nitrogen and carbon
fractionations do not vary much for the first period after the shock. The
observed H$^{13}$CN/HC$^{15}$N can be reproduced by a non-dissociative, C-type
shock with parameters in agreement with previous modelling of L1157-B1. Both
observations and chemical models indicate that the shock propagation does not
affect the nitrogen isotopic ratio that remains similar to that measured in
lower temperature gas in prestellar cores and in protostellar envelopes.
