Try the Ministry of Health, Labour and Welfare:
Imported Foods Inspection Services Home Page
Guidelines on Hygiene Control of Import Processed Foods
There are references to control of metal contamination in there but nothing specific to metal detection I could find:
RECOMMENDED INTERNATIONAL CODE OF PRACTICE GENERAL PRINCIPLES OF FOOD HYGIENE CAC/RCP 1-1969, Rev. 4-2003
5.2.5 Physical and chemical contamination
Systems should be in place to prevent contamination of foods by foreign bodies such as glass or metal shards from machinery, dust, harmful fumes and unwanted chemicals. In manufacturing and processing, suitable detection or screening devices should be used where necessary.
Guidelines on Management and Operation Standards to be Observed by Food-Related Business Operators (Annex to Notice Shoku-an No. 0227012 of February 27th, 2004)
(11) The following items should be implemented wherever possible in manufacturing and processing foods.
(a) Measures should be taken to prevent contamination of raw materials and products by metals, glass, dirt, detergents, and machine oil and other foreign matter, and that inspections should be carried out whenever necessary.
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