Abstract: Provided is a light detection device which allows influence on adjacent pixels to be reduced. The light detection device includes a first substrate portion, a second substrate portion, and a through via. The first substrate portion has a pixel configured to photoelectrically convert incident light. The second substrate portion has a readout circuit configured to output a pixel signal based on charge output from the pixel to a signal line. The through via configured to connect the first substrate portion and the second substrate portion. The pixel has a floating diffusion configured to temporarily retain charge generated by photoelectric conversion. The readout circuit has a first pixel transistor connected to the floating diffusion through the through via and a second pixel transistor connected to the first pixel transistor and the signal line.