A light detecting device includes a case, a light introducing member and a light receiving element. A predetermined light is incident into an inlet face of the introducing member, and an outlet face of the introducing member emits the light incident into the inlet face. A first distance is defined between a top point of the outlet face and a focus of the introducing member, and a second distance is defined between the top point of the outlet face and a light receiving face of the receiving element. The outlet face has a convex lens shape in a manner that the first distance is smaller than the second distance.