This is from the same source as the information in posts 14, 15 and 16.
Section 3 The Coal Industry
Japan had no resources of anthracite, or high-grade coking coal required for steel production, but had reasonable resources of medium- to low-grade bituminous coal. The principal coalfields were in Hokkaido and north-west Kyushu, but there were a few mines in western Honshu producing some fifteen per cent of the total domestic production. In general the main industrial centres in east and central Honshu were not sited close to the coalfields. Normally coal was carried from Hokkaido and Kyushu by sea ports in Honshu and, in 1941, almost the whole of the home output of coal was transported in this way. To economise in coastal shipping and to avoid air raids, a railway tunnel was opened in 1942 under the Shimonoseki Strait, linking the railway system on Honshu with that of Kyushu. A railway ferry was also put into operation across the Tsugaru Strait between Hakodate in Hokkaido and Aomori in Honshu.
In 1937 the coal industry was given a target of seventy million tons a year, which necessitated the greatest possible exploitation of existing mines, the opening up of new mines and a drive to increase production. Large number of uneconomic mines were therefore brought into production, and output rose from forty-five million tons to fifty-seven million tons in 1940, but thereafter fell gradually to forty-nine million tons in 1944 and to some thirty-three million tons in 1945, largely because the army conscripted many of the regular miners who had to be replaced by Koreans, Chinese and prisoners-or-war. By 1945 about two-thirds of the original labour force of trained miners remained and, though the number of men employed in the mines had risen from 300,000 in 1940 to some 420,000 in 1945, output per man had fallen from 173 to 119 tons a year. Production was also affected by difficulties in obtaining mining equipment and by the consequent lowering of the standard of maintenance.
Consumption, which was 51 millions in 1937, rose to 66.5 millions in 1940. During the war years it gradually declined to 52 million tons in 1944, but from June of that year if fell sharply, dropping from 4.5 million tons in that month to 3.5 million in January 1945 and to 2.25 millions in July 1945. This drop occurred despite supplies for the army and navy being maintained at the same level and those for the railways and shipbuilding industry being increased.
Until 1945 consumption always exceeded domestic production. The balance was imported, as long as it was possible, from north China, Inner Mongolia, Manchuria, Indo-China and Formosa. In 1940 imports were some ten millions but they fell to about three millions in 1943. In the first quarter of 1944 monthly imports averaged 416,000 tons, but by the third quarter they had fallen to 190,000 and in the last quarter to 102,000 tons. Thereafter they declined rapidly and ceased altogether in June 1945. Most of the imports were of high-grade copal or coking coal. The fall in imports therefore hit those industries depending on high-grade coal, particularly steel. The railways and coal-burning shipping were able to get their requirements from home production throughout the war. Except for coking coal (for which see The Steel Industry), Japan was never in difficulties over this essential commodity, since the fall in consumption balanced the fall in home production and imports.