I have a rental apartment for investing purpose. If I take money out from this apartment (say, using HELOC against this apartment), is the interest on it tax deductible?

If the purpose of the borrowing is for investment purposes either in the designated property or some other venture then the interest can be deductible against the new investment. If the borrowing is for personal purposes then the interest is not deductible. Create the proper paper trail for CRA if required.