disposed in the portable device through a rf reception block 30 for converting into a digital signal a sampling block 32 for sampling one of the digital signals from the a d converting block 31 a first multiplication block 34 having an xor exclusive or gate circuit for subjecting the sampled digital signal in the sampling block 32 and an output from a first pseudo random number sequence generation block 33 to multiplication block 34 an adding block 35 for adding the outputs of the first pseudo random number sequence generation block 33 for a period of time a threshold value comparing block 36 for comparing an output of the adding block 35 to a preset threshold value to provide a u2018 low u2019 signal if the output of the adding block is greater and a u2018 high u2019 signal if the output of the adding block 35 is smaller and an alarming means 37 for selectively providing an alarm signal according to the signal from the threshold value comparing block 36 referring to fig5 the transmission system in the anti theft circuit of the preferred embodiment of the present invention disposed in the portable device includes a second multiplication block 39 for always multiplying u2018 unity u2019 to an output of the second pseudo random number sequence generation block 38 a d a digital to analog converting block 40 for converting an output of the second multiplication block 39 into an analog value a rf transmission block 41 for transmission of the analog value from the d a converting block 40 in a rf the rf reception block 30 and the.