It is called a Kairo! This was a lacquered box filled with asbestos into which a few coals from the kotatsu (coal heater under the table) were slipped. Or they were heated by sticks of charcoal called kairobai. The burn from this is called kairo yake.
"Another portable fireplace is the smaller Kairo, a metal container in which powdered charcoal is placed. It was used as a bosom, pocket, or belly warmer and is still widely used in Japan today. The modern Kairo, usually carried in coat pockets to keep one's hands warm, is still a metal container (with a thick clothj carrying case) but it is fueled by butane rather than charcoal. Kaori can be purchased anywhere. To activate a hand-warmer just fill the cotton wadding with fluid, light it with a match until it catches and flames, and when the flame dies out replace the lid, put it into its cloth case and pop it into your pocket or a haramaki (belly-wrap or cummerbund). "
http://everything2.com/title/Keeping+Warm+in+JapanThe reason these aren't promoted in being used today is because of the threat of cancer form the asbestos and the charcoal burns - so day they have battery or gas powered ones in much the same shape.
http://linkinghub.elsevier.com/retrieve/pii/S030541790500080X