  A model of a supernova explosion inside a dense extended hydrogenless
envelope is proposed to explain the properties of the light curve for one of
the superluminous supernovae PTF12dam. It is argued in the literature that the
flux of this supernova rises too fast to be explained by the explosion model
due to the instability associated with the electron-positron pair production
(pair-instability supernova, PISNe), but it is well described by the models
with energy input by a magnetar. We show that the PTF12dam-type supernovae can
be explained without a magnetar in a model with a radiative shock in a dense
circumstellar envelope that does not require an excessively large explosion
energy.
