Abstract: A cylindrical frame is provided with an incident angle detecting section for detecting an incident angle of the light from a source of light capable of transmitting pulses and a focus detecting section having a movable lens constructed so that it detects an incident light from the source of light to control the focus position of the incident light. On the basis of the incident angle detected by the incident angle detecting section, the movable lens is caused to be directed to the source of light. Furthermore, by the focus detecting section, the focus position of light is controlled to detect the position of the movable lens. A distance from the position to the source of light is calculated.