Does internal auditor need to have a certificate?

Answer: You will need a certificate that you have passed the internal auditor course if:
1) Your company goes for ISO certification - in that case the certification auditor will ask the internal auditor to prove his knowledge of the standard, and prove his skills for internal auditing.
2) If your company is required by legislation to have certified internal auditor.
3) If you want to prove to your management that you are prepared for this job.

If these three points do not apply to you, then it is not necessary to have a certificate for internal auditor.