Thank you for all the advices you submitted. I will try to resolve the problem with the oil leak sealer first. If that don't solve the problem, then I may have to take it to the shop for a gasket job. The front seal ( I don't know what this is) had been replaced but it did not solve the problem and I just let it leak coz I thought the leak was coming from other part of the engine. I will post the result when the job is done.
Thanks again.