PURPOSE: To facilitate the adjustment, by providing a brush position adjusting electrode pattern in one end part of a variable resistor for information input and using an exposure value indicating means to monitor the contact position of a switching brush.

CONSTITUTION: One and the other sliding brushes which are not shown in a figure are brought into contact with fixed resistances RO and RT and a variable resistance VRSV and slide on sliding parts 53a and a common pattern 54, respectively. When the brush is placed in a position A, the output of an OP amplifier 134 is determined by the resistance ratio of the resistance RT (135) to the resistance RO (132) and is indicated on an exposure indication meter which is not shown in the figure. When the brush is placed in a position B, the brush is brought into contact with patterns 55 and 56, and the output of the OP amplifier 134 is determined by resistances 132, 131, and VRSV (135). When the brush is placed in a position C, the output of the OP amplifier 134 is determined by resistances 132 and 131. Thus, the exposure indication meter is monitored to adjust the brush position easily.