1. A target object information estimating device, comprising: an object detecting unit that outputs a transmission signal of a radar wave every measuring period of time, receives a signal of the radar wave, which is reflected on a target object located on or over a reference surface and is returned to the device through a first transmission path having a first length, and the radar wave which is reflected on the target object and is returned to the device through a second transmission path having a second length different from the first length, and calculates an object distance, from the device to the target object, from the transmission signal and the received signal in each of the measuring periods to detect the target object, the object distance being changed with time; an object tracking unit that tracks the target object detected by the object detecting unit in a tracking term including a plurality of measuring periods; a null distance setting unit that sets the object distance calculated by the object detecting unit, when the target object is tracked by the object tracking unit, as a null distance, each time electric power of the received radar waves is reduced to a minimal value; a modeled null point pattern preparing unit that prepares a modeled null point pattern indicating a modeled pattern of null points for each of a plurality of heights; and an information estimating unit that produces an actual null point pattern indicating a pattern of null points corresponding to the respective null distances set by the null distance setting unit, collates the actual null point pattern with the modeled null point patterns to obtain a collation result, and estimates a height of the target object from the collation result as information about the target object.