I've completed M.Sc (maths). Now a days, I'm teaching in a primary school. I have applied for B.Ed. Is it necessary to join D.El.Ed course?

You have applied for b.Ed, so you are advised to complete it. After completion of B.Ed, you can apply for higher secondary level. Then, you need not do D.El.Ed again. But, if you will apply for D.El.Ed, you have to continue your teaching in elementary level only.