We revisit the effects of an early matter dominated era on gravitational waves induced by scalar perturbations. An early matter dominated era is a period during which the energy density of a massive field dominates the Universe before the reheating. We carefully take into account the evolution of the gravitational potential, source of the gravitational waves, around the transition from an early matter dominated era to the radiation dominated era. As a result, we find that the induced gravitational waves can be suppressed or enhanced depending on the timescales of the transition. This talk will be based on our papers, arXiv:1904.12878 and 1904.12879.