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Total inventories at the end of the month stood at 6.61 million tonnes, according to the Japan Iron & Steel Federation.
This marked a rise of 2.2% from May levels and taking them to their highest levels since September 2012.
Inventories among producers rose by 2.4% to 5.28 million tonnes, while those held by dealers rose for a third straight month, increasing by 1.7% to 1.33 million tonnes.
Stockpiles marked for the domestic market were up by 2.4% to 5.61 million tonnes, while those earmarked for export rose by 1.2% to 1 million tonnes.
A big fall in shipments meant that the inventory ratio, which measures stockpiles against shipments, rose to 110%, with the domestic inventory ratio hitting its highest level in five months at 146%.
The rise in stockpile levels has caused senior industry officials again to issue a warning to producers to make sure they keep production in line with actual demand.
On the other hand, other officials have said that the inventory ratio is bound to increase with the outlook for further pick-up in demand in the coming months looking positive, and that the pile-up has yet to reach danger levels.
Shipments in June fell 6.6% from May to 6.01 million tonnes as exports decreased by 10.2% on the month to 2.17 million tonnes. Shipments to the domestic market, meanwhile, retreated by 4.4% to 3.84 million tonnes.
This far exceeded the 4.4% decrease in output to 6.01 million tonnes.