A rotary encoder comprising: a rotary scale having a spiral pattern, the center of which is located at the rotation axis of the rotary scale, the radius of which continuously changes in the rotational direction of the rotary scale, the width and pitch of which are constant in the radial direction of the rotary scale, and the reflectance or transmittance of which changes in the rotational direction; and a signal processor, wherein: a first light-receiving part is constituted by a first sensor array group including at least two sets of sensor arrays arranged at mutually different radial position, a first reading area on the rotary scale wherein the first sensor array group are arranged to read the spiral pattern including at least one cycle of the spiral pattern in the radial direction over an entire rotational angle range of the rotary scale, the first sensor array group is arranged to output sinusoidal signals corresponding to a radial cycle of the spiral pattern, the signal processor is arranged to produce a first phase signal from two phase sinusoidal signals produced based on the sinusoidal signals output from the first sensor array group, and the signal processor is further arranged to produce, based on the first phase signal, a rotational angle signal defining a rotational angle of the rotary scale characterized by: a sensor unit disposed so as to face the rotary scale and including a light source arranged to illuminate the spiral pattern, and the first light-receiving part arranged to detect light incident thereon via the spiral pattern; the spiral pattern being radially displaced at a constant angular cycle, and an integral multiple of the constant angular cycle being equal to a circumferential length of the first reading area on the spiral pattern for a circumferential length of the sensor array in the first sensor array group. a rotary scale having a spiral pattern, the center of which is located at the rotation axis of the rotary scale, the radius of which continuously changes in the rotational direction of the rotary scale, the width and pitch of which are constant in the radial direction of the rotary scale, and the reflectance or transmittance of which changes in the rotational direction; and a signal processor, wherein: a first light-receiving part is constituted by a first sensor array group including at least two sets of sensor arrays arranged at mutually different radial position, a first reading area on the rotary scale wherein the first sensor array group are arranged to read the spiral pattern including at least one cycle of the spiral pattern in the radial direction over an entire rotational angle range of the rotary scale, the first sensor array group is arranged to output sinusoidal signals corresponding to a radial cycle of the spiral pattern, the signal processor is arranged to produce a first phase signal from two phase sinusoidal signals produced based on the sinusoidal signals output from the first sensor array group, and the signal processor is further arranged to produce, based on the first phase signal, a rotational angle signal defining a rotational angle of the rotary scale characterized by: a sensor unit disposed so as to face the rotary scale and including a light source arranged to illuminate the spiral pattern, and the first light-receiving part arranged to detect light incident thereon via the spiral pattern; the spiral pattern being radially displaced at a constant angular cycle, and an integral multiple of the constant angular cycle being equal to a circumferential length of the first reading area on the spiral pattern for a circumferential length of the sensor array in the first sensor array group. a first light-receiving part is constituted by a first sensor array group including at least two sets of sensor arrays arranged at mutually different radial position, a first reading area on the rotary scale wherein the first sensor array group are arranged to read the spiral pattern including at least one cycle of the spiral pattern in the radial direction over an entire rotational angle range of the rotary scale, the first sensor array group is arranged to output sinusoidal signals corresponding to a radial cycle of the spiral pattern, the signal processor is arranged to produce a first phase signal from two phase sinusoidal signals produced based on the sinusoidal signals output from the first sensor array group, and the signal processor is further arranged to produce, based on the first phase signal, a rotational angle signal defining a rotational angle of the rotary scale a sensor unit disposed so as to face the rotary scale and including a light source arranged to illuminate the spiral pattern, and the first light-receiving part arranged to detect light incident thereon via the spiral pattern; the spiral pattern being radially displaced at a constant angular cycle, and an integral multiple of the constant angular cycle being equal to a circumferential length of the first reading area on the spiral pattern for a circumferential length of the sensor array in the first sensor array group.