Japanese Service Flags, 1900s

This was in use from 7 October 1889 through the end of World War II.
Nozomi Kariyasu, 22 February 2000

A similar flag is shown for 1925 with different ratios
Jaume Ollé, 24 October 1999

This is shown in a 1910 flag plate. Later, the flag was retired and a traditional western style red cross was used
Jaume Ollé, 24 October 1999

The Army transport vessel's flag was used from 21 May 1900 to the end of World War II.

Source: Flaggenbuch 1925

The white stands for sky, blue for sea and the red disc symbolizes the sun rising from the horizon.
Nozomi Kariyasu, 25 October 1999

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The flag was adopted 3 August 1892 and is still in use
Nozomi Kariyasu, 22 February 2000

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The flag was adopted on Aug 3rd 1892 originally for ensign but now hoisted at customs office buildings as well .The official gazette says the flag proportion be 2:3. The disc diameter is 2/3 of width. The white stands for sky, blue for sea and red for rising sun from the sea.

Source:
Flaggenbuch 1925
Flaggenbuch 1939 [neu39]

See also:

• Customs flag (Japan)

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The red disc of the flag image from Flaggenbuch 1925 is too large. My source are Japanese flag books from 1910 and 1995 which show the disc size is 3/5 of length and the Customs Flag is still in use now in Japan.
Nozomi Kariyasu, 24 October 1999