1. An image processing apparatus which imposes a usage restriction on a function included in said image processing apparatus according to access control information for each user, comprising: a setting unit which sets a condition required to release a usage restriction of a function; a determination unit which determines whether or not the condition required to release the usage restriction is satisfied when a request to use a usage-restricted function according to the access control information is received; a generation unit which generates, when said determination unit determines that the condition required to release the usage restriction is satisfied, restriction release information required to temporarily release a restriction on the usage-restricted function; a first issuance unit which issues the generated restriction release information as a response to the request; an execution unit which releases, when the issued restriction release information and a processing request regarding the usage-restricted function are received, the restriction on the usage-restricted function according to the access control information, based on the restriction release information, and executes processing corresponding to the processing request by using the usage-restricted function; and a discarding unit which discards the restriction release information based on a discarding condition after said execution unit executes the processing, wherein the determination unit determines whether or not the condition is satisfied based on whether an application included in the request and the usage-restricted function match the condition.