A collision preventing device includes an acceleration/deceleration simulating unit 30 that performs acceleration/deceleration processing based on a moving command generated by a function generation unit 16 according to a method similar to that used by acceleration/deceleration units 18x and 18z and obtains a moving path resulting from the acceleration/deceleration processing, an interference check unit 34 that performs interference check between a mobile member and an interfering object along the moving path generated by the acceleration/deceleration simulating unit 30 and determines whether any interference may occur, a delay unit 32 that successively stores moving commands generated by the function generation unit 16 and successively outputs a moving command having been stored a predetermined time before, and a moving command blocking unit 36 that sends the moving command output from the delay unit 32 to the acceleration/deceleration units 18x and 18z if the interference check unit 34 determines that there is no interference, and blocks the moving command to be sent from the delay unit 32 to the acceleration/deceleration units 18x and 18z if the interference check unit 34 determines that interference may occur.