This nifty gadget is for making okayu (watered down rice) for babies.
The glass jar goes in your automatic rice maker when you make normal rice. The rice to water proportion is different from the regular rice making for a softer cooked rice which babies can eat. You start out at 10/1 at 6 months and by age one the babies should be able to eat regular rice. The various measurements are written right on the jar.
The other pieces in the picture include a strainer, measuring spoon and a little rubber "pot holder" for taking the jar out of the rice maker as it is very, very hot after cooking. I'm sure I'll misplace the instructions at some time too so I took a picture of them.
While we don't eat rice every day we have found this to be a very helpful tool. I often would make a jar full and then divide it into smaller containers to have on hand for quick meals. The little guy liked rice mixed in with just about every fruit or veggie mush that we gave him.