An audio apparatus having a detachable control panel or operating-part main unit includes a microcomputer for detecting an OFF state of an ACC position of the ignition key, or the attached/detached state of the operating-part main unit. When such states are detected, normal operations of the microcomputer; namely, the operations to turn on an SWD V.sub.DD terminal and an A/D port and to set a KEY DATA terminal into a standby mode, are stopped intermittently in order to execute a low consumption process in which electric power is consumed only for holding information in memory in the microcomputer. When the microcomputer detects the OFF state of the ignition key or the OFF state of an installation detection switch and, an alarm ON state is set in the memory in the microcomputer, then an alarm sound is generated from a speaker for a predetermined time. When the alarm sound is unnecessary, the state of the memory in the microcomputer can be updated to an alarm OFF state by depressing a function button and operating the ignition key. When the installation detection section detects the removal of the operating-part main unit, on the basis of a decrease in voltage value of an installation detection switch section and CPU recognizing the removal of the operation-part main unit, a control section sends control signals to a power supply limiting circuit and a data transmission switch section.