According to an aspect, a detection device includes: photodiodes; a light source; a detection circuit configured to detect detection signals output from the photodiodes; and an exposure period adjusting circuit configured to determine an exposure period of the photodiodes. A relation between an output value output from the detection circuit and the exposure period comprises a linear region and a saturated region, and a gradient of the output value with respect to the exposure period in the saturated region is smaller than a gradient of the output value with respect to the exposure period in the linear region. The exposure period adjusting circuit is configured to determine the exposure period based on a first output value output from the detection circuit in a first exposure period, a second output value output from the detection circuit in a second exposure period, and a reference output value set in advance.