To prevent a vehicle in an adjacent lane from being erroneously mistaken for a preceding vehicle while increasing capturing property with respect to a preceding vehicle, by improving the setting and application of self-lane decision width in a radar device for certifying the preceding vehicle, and for outputting the information to a control unit.

This on-vehicle radar device is provided with a means (S1) for defining a vehicle as a target, and for detecting the information; means (S2, S3, S4, S8, S9) for deciding whether or not the target has existed on a self-lane decision width L by applying wide self-lane decision width L2to the target information which is being certified as a preceding vehicle, and for deciding whether or not the target has existed on the self-lane decision width line L by applying narrow self-lane decision width L1to the other target information; and means (S6, S7) for certifying the target whose inter-vehicle distance with its own vehicle is the shortest on the self-vehicle decision width L as the preceding vehicle from the decision result.

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