A scale is provided with a two-level code pattern according to a pseudo random code sequence along a length measurement direction. Each code of the two-level code pattern indicates a code “1” or “0”, each code includes two bits, and each bit of the two bits is L or H. The code “1” is represented by an A pattern which is a combination of L and H, and the code “0” is represented by a B pattern which is a combination of L and L or by a C pattern which is a combination of H and H. When the codes “0” are continued, the B pattern and the C pattern are alternately used. The scale is commonly used in a reflective type encoder or transmissive type encoder. A detection head part includes an inversion processing unit which performs inversion processing to a detection image of the scale as required.