Whenever he use the computer or browse something on his phone, he deletes it asap. I don't know why he do that. And its been 5years we are together.

Why are you checking?

Wow, your response wondergirl, is perfect! :) God bless you!

Now this sound so familiar. It happened to me as well but I only checked my boyfriend. History because his attitude had changed. I started noticing red flags. As soon as I was able to see what he was doing I saw numerous emails to woman who had personals on Craigslist. He was even responding to girls on dating sites. I was devastated but I was happy to know the truth. Now in your situation I really don't know all the details but there must be a reason why you checked. A man isn't going to delete his search history if he wasn't hiding something. If there's some suspicion of cheating look for the signs. Such as is he more into his appearance. Does he act distant, less affectionate. Does he hold on to his cell phone and doesn't let you near it or allow you to see it. Or if he has a lock that he didn't have one before. The most concerning sign is a change in your sex life. Is he still interested in sex or he hardly wants to anymore.
But I suggest you talk to him first explaining your concerns and that its making you feel uncomfortable. His reaction should tell you everything. If he starts to get angry and aggressive then he's up to no good. If his reaction is cool and collected then you have nothing to worry about. Communication is very important in a relationship, but most of all trust is the key to a successful relationship. Without it the relationship is doomed. GOOD LUCK

He may do it because he knows you look, and he does not want to have to explain any simple phone call, or everything he does. He may enjoy porn but does not want to fight about it with you??