Is the ceramic steamer meant to be placed over a gas flame to steam like a pot, or is it exclusively for the microwave?

It can go into the microwave and dishwasher, but it must stop before a gas flame and oven^[1][2]. The ceramic steamer from Nakagawa Masashichi Shoten cannot be used over direct flames such as a gas stove or radiant cooktop, nor can it be used in an oven or on induction^[3]. Generally, unless it is heat-resistant ceramic specifically crafted for direct flame use, heat concentrates where the flame directly touches, risking cracking or breaking the vessel.

It cooks with microwaves instead of fire. It works by heating the moisture contained in the ingredients inside the microwave to steam them. When heating is finished, you can slightly shift the lid open to cleanly drain the water at the bottom through the opening along the vessel's rim^[4]. There is no need to transfer the food to another plate, either. You can enjoy it by placing the steamer directly on the table, and once the meal is over, conveniently wash it by putting it in the dishwasher^[5].