Image reading device and image forming apparatus for detecting that document reading unit is positioned at document reference line

An image reading device, comprising:

• a housing including a planar first housing member having a document reading area on which a document is placed and a rectangular second housing member attached to the first housing member;
• a document reference line formed on the first housing member to position the document placed on the document reading area in a sub-scanning direction of the housing;
• a document reading unit disposed inside the housing, a bottom portion of the document reading unit including two slits with a partition formed therebetween, the document reading unit configured to start reading the document, through the document reading area while moving in a sub-scanning direction of the housing, based on a start signal indicating that the document reading unit is positioned at the document reference line;
• a reference position detector including a light receiving portion configured to receive light emitted from a light emitting portion, the reference position detector positioned on a plane surface of a square that extends perpendicular to the document reading area along a main scanning direction of the document reference line such that when the document reading unit is positioned at the document reference line, the partition blocks the emitted light from being received at the light receiving portion to detect that the document reading unit is positioned at the document reference line;
• and
• a controller configured to transmit the start signal to the document reading unit, if the reference position detector detects that the document reading unit is positioned at the document reference line.