Dear Hans and Otared,

Thank you for the reply.

 I tried "%!TEX root =“  with no space before but it does’t work.

I think that ConTeXt doesn’t do anything as you said.

The strange thing is that  it worked in ConTeXt file sometime ago. 
I don’t remember from when it didn’t work.

Anyway it is not a big problem, though.

Best regards,

Dalyoung