The present invention provides an FSK demodulator and an inflection point method for extracting a greater amount of effective inflection points of a frequency detection signal while reducing erroneous detection of the inflection points. The inflection point detection circuit of a frequency shifting and removing portion includes an inflection point extraction part to extract the inflection point corresponding to variation of a sample value of an amplitude value of the frequency detection signal; an amplitude determination part to determine if a size between peak values of sample values in front and rear of the inflection point exists in a first predetermined range; a preamble determination part to determine if a difference between initial and final sample values of at least one of a symbol having the extracted inflection point and a right before symbol exists in a second predetermined range; and an AND operation part, wherein when the amplitude determination part determines the size between peaks in a first regulated range and the preamble determination part determines difference in a second regulated range, the inflection point extracted by the inflection point extraction part is used as a normal inflection point.