Recently, motor vehicle theft and on-board thefts of vehicles have increased, and a burglar alarm system in a vehicle that goes off loudly when a suspicious person intrudes a vehicle that is parked has been widely used.  The burglar alarm system for a vehicle includes a moving object detecting apparatus for detecting an existence of a moving object (or person) in a monitoring space around a vehicle (see, e.g., Japanese Patent Application Publication No.  2008-151506). 
FIG.  17 shows an example of a conventional moving object detecting apparatus described in the above Patent.  In the patent, a transmitter 1003 is driven by a transmission signal of a certain frequency (i.e., an oscillation frequency) at which an oscillation circuit 101 oscillates, and radiates an ultrasonic wave of the same frequency as the oscillation frequency of the oscillation circuit 1 into a monitoring space.  A receiver 1004 receives a reflective ultrasonic wave from an object O present in the monitoring space.  The receiver 1004 converts the received reflective wave into a reception signal Ein, and the reception signal Ein is inputted to each of first and second phase detection circuits (mixers) 106A and 106B and mixed with reference signals E03 and E04 having the same frequency as the oscillation frequency of the oscillation circuit 1, respectively. 
FIGS.  18A to 18C show waveform views of the transmission signal E01, and the reference signals (mixer signals) E03 and E04, respectively.  The reference signal E04 equals to the transmission signal E01 and, therefore, the reference signal E04 and the transmission signal E01 are in phase. 
Meanwhile, the reference signal E03 is based on a signal having a phase delayed from that of the transmission signal E01 by 90 degrees (π/2) by a phase-shifting circuit 110 and, therefore, the reference signals E03 and E04 are out of phase.  Thus, the phases of a pair of Doppler signals E1 and E2 outputted as a beat signal from the first and second mixer circuits 106A and 106B are different from each other.  The pair of Doppler signals E1 and E2 are amplified by first and second amplifying circuits 113A and 113B, respectively, and then inputted to a signal processing unit 108. 
In the signal processing unit 108, a sampling circuit 185 samples the pair of Doppler signals E1 and E2 at a predetermined sampling frequency and quantizes the sampled signals to convert an analog value into a digital value.  The converted digital values are sequentially stored in a non-volatile memory 181.  Here, assuming that a digital value (digital data) converted from the Doppler signal E1 by the sampling circuit 185 is Xn and a digital value (digital data) converted from the Doppler signal E2 by the sampling circuit 185 is Yn (where n is a positive integer), a vector Rn is defined in which the origin of a two-dimensional orthogonal coordinate system serves as a starting point and (Xn, Yn) serves as an ending point.  Also, the magnitude of the vector Rn depends on amplitudes of the Doppler signals E1 and E2. 
Further, a vector rotation angle calculation circuit 186 of the signal processing unit 108 calculates an angle Φn (hereinafter, referred to as “a rotation angle of the vector”) between a vector Rn-1 which has been obtained from an immediately previous sampling that was stored in the memory 181 and the vector Rn obtained from the current sampling.  The vector rotation angle calculation circuit 186 calculates the rotation angle Φn by the following equation:Φn=arctan {(Xn-1Yn−Yn-1Xn)/(Xn-1Xn+Yn-1Yn)}  Eq. ( 1)
Thus, when the object O approaches the detecting apparatus, the vector Rn rotates counterclockwise and a polarity of the rotation angle Φn becomes positive.  Further, when the object O moves away from the detecting apparatus, the vector Rn rotates clockwise and the polarity of the rotation angle Φn becomes negative. 
The rotation angle Φn obtained by the vector rotation angle calculation circuit 186 is added up (or integrated) by a cumulative addition unit 187, and the added value (=Φ1+Φ2+ . . .  +Φn+ . . . )  is proportional to a movement distance of the object O. In the signal processing unit 108, the cumulative value added up by the cumulative addition unit 187 is compared with a threshold by a comparison unit 188.  When the added value exceeds the threshold value, the comparison unit 188 outputs a detection signal.  The detection signal is inputted to an alarm driving circuit 111 and, accordingly, the presence of the moving object O is appropriately notified by an alarm (not shown)
However, in the conventional example, since the transmitter 1003 for radiating ultrasonic waves is separated from the receiver 1004 for receiving reflective waves, it requires a large space to install the transmitter 1003 and the receiver 1004.  Especially, when the transmitter 1003 and the receiver 1004 are separately installed in a vehicle, it is difficult to secure an installation place.