A fatigue testing apparatus for testing the fatigue of a test piece, the apparatus comprising: a vibrator means and a sensor configured to measure vibration acceleration signals, the vibrator means being configured to continuously and repeatedly impart a bending deformation to the test piece and fixable to the test piece at one end; an acceleration sensor attachable to the test piece at an end opposite to the vibrator means; and a strain acquiring means configured to directly measure or calculate a strain, ε, of a fatigue-strength evaluation site of the test piece in a preliminary test, the apparatus being characterized in that: the sensor and vibrator means are configured to follow a change of a natural frequency, f, of the test piece and impart the followed natural frequency to the test piece, and a control device within the apparatus configured to control the vibrator means, to monitor the phase difference between the vibration acceleration signals and response acceleration signals for changing the vibration frequency of the vibrator means based on feedback control which causes the phase difference to be 90°, and to calculate a stress amplitude, σ, at the fatigue-strength evaluation site, by using: the strain acquired by the strain acquiring means, and a response acceleration, a, obtained from the acceleration sensor in the preliminary test, in equation 1: σ = εE = c 2 a 2 πf 2 ⋅ f 2 c 1 = c 3 a where E is the rigidity of the test piece or the bend elastic coefficient of the test piece acquired in a test separate to the preliminary test, c 1 is a first proportional constant, c 2 is a second proportional constant, and c 3 is a third proportional constant acquired in the preliminary test; wherein the strain acquiring means is configured to estimate the strain, ε, of the fatigue-strength evaluation site by using a displacement amplitude at a position of the acceleration sensor in the equation: ε = c 2 d where d, the displacement amplitude, is calculated using the equation: d = a 2 πf 2. a vibrator means and a sensor configured to measure vibration acceleration signals, the vibrator means being configured to continuously and repeatedly impart a bending deformation to the test piece and fixable to the test piece at one end; an acceleration sensor attachable to the test piece at an end opposite to the vibrator means; and a strain acquiring means configured to directly measure or calculate a strain, ε, of a fatigue-strength evaluation site of the test piece in a preliminary test, the apparatus being characterized in that: the sensor and vibrator means are configured to follow a change of a natural frequency, f, of the test piece and impart the followed natural frequency to the test piece, and a control device within the apparatus configured to control the vibrator means, to monitor the phase difference between the vibration acceleration signals and response acceleration signals for changing the vibration frequency of the vibrator means based on feedback control which causes the phase difference to be 90°, and to calculate a stress amplitude, σ, at the fatigue-strength evaluation site, by using: the strain acquired by the strain acquiring means, and a response acceleration, a, obtained from the acceleration sensor in the preliminary test, in equation 1: σ = εE = c 2 a 2 πf 2 ⋅ f 2 c 1 = c 3 a where E is the rigidity of the test piece or the bend elastic coefficient of the test piece acquired in a test separate to the preliminary test, c 1 is a first proportional constant, c 2 is a second proportional constant, and c 3 is a third proportional constant acquired in the preliminary test; wherein the strain acquiring means is configured to estimate the strain, ε, of the fatigue-strength evaluation site by using a displacement amplitude at a position of the acceleration sensor in the equation: ε = c 2 d where d, the displacement amplitude, is calculated using the equation: d = a 2 πf 2. where E is the rigidity of the test piece or the bend elastic coefficient of the test piece acquired in a test separate to the preliminary test, c 1 is a first proportional constant, c 2 is a second proportional constant, and c 3 is a third proportional constant acquired in the preliminary test; wherein the strain acquiring means is configured to estimate the strain, ε, of the fatigue-strength evaluation site by using a displacement amplitude at a position of the acceleration sensor in the equation: ε = c 2 d where d, the displacement amplitude, is calculated using the equation: d = a 2 πf 2.