1. An image forming apparatus having hardware resources including an operation accepting portion to accept an instruction operation designated by an operation user from among a plurality of kinds of operations, and a control portion to control said hardware resources, said control portion comprising: an internal control portion operable, in response to an input of an internal command, to execute a process predetermined for said input internal command in order to control said hardware resources; a first application executing portion to execute an application switching program; a plurality of second application executing portions each configured to execute a corresponding customization application program different from said application switching program; and an application control portion operable, in the case where one of said plurality of second application executing portions outputs an application command which has been released for controlling said internal control portion, to convert said output application command into said internal command and output said obtained internal command to said internal control portion, said application control portion including a current task setting portion to set one of said plurality of second application executing portions to a currently selected state in which the portion is capable of receiving a common operation predetermined from among a plurality of operations acceptable by said operation accepting portion, and an arbitration portion operable, in the case where said common operation is accepted by said operation accepting portion, to output operation identification information for identifying the common operation to the one of said plurality of second application executing portions that has been set in the currently selected state; said first application executing portion including a post-transition application determining portion operable, in the case where said operation accepting portion accepts a process switching operation predetermined, besides said common operation, from among the plurality of operations acceptable by the operation accepting portion, to determine one of said plurality of second application execution portions that should be changed to the currently selected state next, on the basis of the one of said plurality of second application executing portions being set in the currently selected state and on the basis of said accepted process switching operation, said current task setting portion setting said second application executing portion determined by said post-transition application determining portion to the currently selected state.