This patent specification describes a relative position detection device and a detector for a rotary body and an image forming apparatus including the relative position detection device. The relative position detection device includes a scale comprising a pattern continuously formed by changing reflectance or transmittance and moving in a direction, a light source to irradiate the scale, a light receiving unit to read the pattern and output a signal, and a signal processing unit to process the output signal of the light receiving unit. The pattern is formed by a line pattern having regularly spaced lines perpendicular to the direction of movement of the scale, the light receiving unit is slanted with respect to the lines, and the relative position detection device detects a relative position change and speed change of the scale.