To provide an electronic component unit which is readily assembled in one operation without an increase in the number of components and man-hour, prohibits disassembling of the unit after assembling to prevent illegal modification, and leaves traces of illegal modification once the unit is illegally modified.

The electronic component unit 10 includes a circuit board 13 wherein an electronic component 16 with a radio wave transmission function and a connection terminal 17 are mounted, a battery holder 12 holding a battery connected to the connection terminal 17 of the circuit board 13, and a holder cover 11 covering the battery holder while holding the battery holder 12 and the circuit board 13 in between. The connection terminal 17 is formed with a first and second engaging portions 17a, 17b to be engaged with the battery holder 12 and the holder cover 11 respectively.

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