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In the conventional view of type II migration, a giant planet migrates inwardin the viscous velocity of the accretion disc in the so-call disc-dominatecase. Recent hydrodynamic simulations, however, showed that planets migratewith velocities much faster than the viscous one in massive discs. Such fastmigration cannot be explained by the conventional picture. Scardoni et al.(2020) has recently argued this new picture. By carrying out similarhydrodynamic simulations, they found that the migration velocity slows downwith time and eventually reaches the prediction by the conventional theory.They interpreted the fast migration as an initial transient one and concludedthat the conventional type II migration is realised after the transient phase.We show that the migration velocities obtained by Scardoni et al. (2020) areconsistent with the previous simulations even in the transient phase that theyproposed. We also find that the transient fast migration proposed by Scardoniet al. (2020) is well described by a new model of Kanagawa et al. (2018). Thenew model can appropriately describe significant inward migration during theinitial transient phase that Scardoni et al. (2020) termed. Hence, we concludethat the time-variation of the transient migration velocity is due to thechanges of the orbital radius of the planet and its background surface densityduring the migration.

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