PURPOSE: To detect an obstacle certainly over a wide range of distance by providing a hologram optical element for splitting a laser beam into a plurality of beams having different divergence angle.

CONSTITUTION: When a laser light 23 emitted from a semiconductor laser 27 at a light transmitting section 9 impinges on a hologram dry plate 31, the laser light 23 is split into a luminous flux 37 having a divergence angle θ1 and a luminous flux 39 having a divergence angle θ2 spreading annularly around the flux 37. The luminous flux 37 has a long range and irradiates a remote obstacle whereas the luminous flux 39 has a large divergence angle θ2 and irradiates a close obstacle on the side part. The reflected light from an obstacle is received at a light receiving section and shaped and then a control section measures the time being elapsed after irradiation of the laser light 23 before arrival of the reflected light thus determining the distance to the obstacle. The dry plate 31 is produced by partially masking a translucent hologram dry plate, irradiating the dry plate with a laser light at a specified divergence angle to cause interference with the light from same light source and then recording a hologram thus formed. The operation is performed two times for the divergence angles θ1 and θ2 to produce the dry plate 31.