insertion hole 163 the button shaft 151 a inserted into the shaft insertion hole 163 is connected to the push shaft 121 a of the rotary switch 121 in the shaft insertion hole 163 thus the user can operate the rotary switch 121 by rotating the rotary knob 160 or pushing the push button 151 thereby selecting a desired function the knob body 161 of the rotary knob 160 is seated on the base panel 141 of the knob guide 140 and the boss 162 of the rotary knob 160 passes through the base panel 141 and is extended downwardly the knob guide 140 further includes a boss insertion part 170 which restricts the boss 162 to prevent the rotary knob 160 from shaking since the boss insertion part 170 prevents the rotary knob 160 from shaking as described above when the rotary knob 160 and the knob guide 140 contact each other an area of friction therebetween is reduced that is when a conventional rotary knob rubs against surrounding parts the circumferential surface of the body of the rotary knob rubs against the surrounding parts however in the present embodiment the boss 162 which has a relatively small diameter rubs against the boss insertion part 170 and thus noise and malfunction caused by the friction are reduced the boss insertion part 170 is provided with an extension 170 a extended upwardly from the base panel 141 of the knob guide 140 a boss insertion hole 171 into which the boss 162 of the rotary knob 160 is inserted is formed within the extension 170 a although this embodiment illustrates the extension 170 a extended.