Who audits the books of accounting companies?

I did my training at a medium sized firm, and their books were audited by another mid sized audit firm.

I have also worked at a small practice who audits the books of some of the group of companies in the large audit firms.

I can say without doubt that the examination and training according to the auditing statements and standards, there is an acceptance of professionalism and one audit firm can appoint another registered auditor without concern of unethical behavior. 

There is also the associated and linked firms which would be safer option in the choice of registered auditor appointments.

I would be interested to follow the question and get an opinion of others.  

Answer is Very Clear and Simple 

---    An AUDITOR

it is depend upon the size and nature of the business firm.

 - Some firm appoint Statutory auditor ( business firm who get high turn over) 

 - Manufacturing companies appoint COST AUDITOR

 - Small firms appoint internal staff as their auditor

Statutory Auditors audits the books of accounting companies.

agree with answers >>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<

agreed with all ....................................

The question is not clear .................

An external Auditor..........................