A position detecting device (1000, 1300, 1500, 1600) comprising: a plurality of detecting units (6a, 6b, 6c) that faces a mark-formation area.of an object where marks are formed at predetermined intervals, and detects the marks at detection positions while the object is moving; a plurality of housing units (1011, 1012, 1311, 1312, 1511, 1512, 1513, 1611, 1612, 1613) each housing one of the detecting units (6a, 6b, 6c); and a holding member (1005, 1305, 1505, 1605) that fixedly holds the housing units (1011, 1012, 1311, 1312, 1511, 1512, 1513, 1611, 1612, 1613) at fixed positions (1021, 1022, 1321, 1322, 1521, 1522, 1523, 1621, 1622, 1623) wherein the plurality of housing units includes a first housing unit and a second housing unit, the plurality of detecting units includes a first detecting unit that is housed in the first housing unit and detects the marks at a first detection position of the detection positions; and a second detecting unit that is housed in the second housing unit and detects the marks at a second detection position of the detection positions, the fixed positions include a first fixed position that is located between the first detection position and a side edge of the first housing unit opposite to a side edge facing the second housing unit; and a second fixed position that is located between the second detection position and a side edge of the second housing unit opposite to a side edge facing the first housing unit, or a second fixed position that is located between the second detection position and a side edge of the second housing unit facing the first housing unit, characterized in that the housing units (1011, 1012) and the holding member satisfy a relation: - CyL ⁢ 1 ≤ yL ⁢ 2 - x ⁢ d ⁢ 1 + d ⁢ 2 ≤ CyL ⁢ 1, when the second fixed position is located between the second detection position and a side edge of the second housing unit opposite to a side edge facing the first housing unit, or - CyL ⁢ 1 ≤ yL ⁢ 2 - x ⁢ d ⁢ 1 + d ⁢ 2 ≤ CyL ⁢ 1, where the second fixed position is located between the second detection position and a side edge of the second housing unit facing the first housing unit, where x is the expansion coefficient of the housing units (1011, 1012), y is the expansion coefficient of the holding member, L1 is a distance between the first detection position and the second detection position, L2 is a distance between the first fixed position and the second fixed position, d1 is a distance between the first fixed-position plane and the first detection-position plane, d2 is a distance between the second fixed-position plane and the second detection-position plane, the fixed-position planes including one of the fixed positions and being perpendicular to the moving direction of the object, and the detection-position planes including one of the detection positions and being perpendicular to the moving direction of the object, and C is a constant that satisfies 0≤C≤1. a plurality of detecting units (6a, 6b, 6c) that faces a mark-formation area.of an object where marks are formed at predetermined intervals, and detects the marks at detection positions while the object is moving; a plurality of housing units (1011, 1012, 1311, 1312, 1511, 1512, 1513, 1611, 1612, 1613) each housing one of the detecting units (6a, 6b, 6c); and a holding member (1005, 1305, 1505, 1605) that fixedly holds the housing units (1011, 1012, 1311, 1312, 1511, 1512, 1513, 1611, 1612, 1613) at fixed positions (1021, 1022, 1321, 1322, 1521, 1522, 1523, 1621, 1622, 1623) wherein the plurality of housing units includes a first housing unit and a second housing unit, the plurality of detecting units includes a first detecting unit that is housed in the first housing unit and detects the marks at a first detection position of the detection positions; and a second detecting unit that is housed in the second housing unit and detects the marks at a second detection position of the detection positions, a first detecting unit that is housed in the first housing unit and detects the marks at a first detection position of the detection positions; and a second detecting unit that is housed in the second housing unit and detects the marks at a second detection position of the detection positions, the fixed positions include a first fixed position that is located between the first detection position and a side edge of the first housing unit opposite to a side edge facing the second housing unit; and a second fixed position that is located between the second detection position and a side edge of the second housing unit opposite to a side edge facing the first housing unit, or a second fixed position that is located between the second detection position and a side edge of the second housing unit facing the first housing unit, a first fixed position that is located between the first detection position and a side edge of the first housing unit opposite to a side edge facing the second housing unit; and a second fixed position that is located between the second detection position and a side edge of the second housing unit opposite to a side edge facing the first housing unit, or a second fixed position that is located between the second detection position and a side edge of the second housing unit facing the first housing unit,