Tire pressure detection side receiving circuit and tire monitor system provided therewith

A tire pressure detection side receiving circuit (80) includes an antenna resonance circuit (81) having an antenna (40), a capacitor (C1 to C4) and a coil (L1), a capacitance changing circuit (82) for changing a capacitance of the capacitor, an oscillation circuit (85) capable of imparting a test wave to the antenna resonance circuit (81), a resonance level detector (83) capable of detecting a resonance level of the antenna resonance circuit (81), a memory (34) storing preset reference frequency, a resonance frequency detector (S2, 31) changing a frequency of the test wave to be imparted to the antenna resonance circuit (81) while the capacitor (C1 to C4) of the antenna resonance circuit (81) is maintained at a constant capacitance, thereby detecting as a measured resonance frequency a frequency in a case where an amplitude level of the test wave has reached a peak, and a tuning control (S3, 31) controlling the capacitance changing circuit (82) and changing the capacitor capacitance so that the measured resonance frequency is approximated to a reference frequency.