Thanks.

Now the leak became worse.
The coolant level dropped half an inch in one month.

A mechanic suggested replacing water pump and thermostat cover.
Where are these components under the hood?
I want to "see" the leak location before spending money.

By the way, I ran 313 miles (60~80% highway miles) since last diesel fill up. Yesterday, I filled 12.562 galons.
So this results in ~25 mpg.
When I checked mpg last fall, it was about 31 mpg.
I wonder how my car became so worse in half a year.
I do not feel strange sound or vibration from the engine.
The coolant temperature is usually 85~90 degrees C.

My car is 1992 300D 2.5 turbodiesel. 229982 miles.