A method for detecting a tire having a decreased internal pressure by making a relative comparison of wheel speeds of tires attached to the respective wheels of a vehicle, comprising: a step of detecting wheel rotation information of the respective tires of the vehicle; a step of calculating wheel speeds based on the detected wheel rotation information; a step of calculating a decreased pressure determination value by making a relative comparison of the calculated wheel speeds; and a step of determining a tire having a decreased internal pressure when the resultant decreased pressure determination value exceeds a predetermined threshold value, characterized in that the method further comprises the following steps of setting the predetermined threshold value: determining in a calibration stage a proportional constant between a load sensitivity and a decreased pressure sensitivity during vehicle turning and storing the proportional constant in a storage means; estimating in an initialization stage a load sensitivity during vehicle turning of an actual running and multiplying the estimated load sensitivity with the proportional constant, stored in advance in the storage means, to thereby automatically set the predetermined threshold value, wherein the load sensitivity is calculated based on a relational expression between a ratio of the dynamic loaded radius change due to the load shift during turning to the dynamic loaded radius, the ratio being calculated based on a yaw rate and a lateral acceleration obtained from a yaw rate sensor and a lateral acceleration sensor mounted in the vehicle and wheel speeds of left and right wheels of the vehicle, and a lateral acceleration. a step of detecting wheel rotation information of the respective tires of the vehicle; a step of calculating wheel speeds based on the detected wheel rotation information; a step of calculating a decreased pressure determination value by making a relative comparison of the calculated wheel speeds; and a step of determining a tire having a decreased internal pressure when the resultant decreased pressure determination value exceeds a predetermined threshold value, characterized in that the method further comprises the following steps of setting the predetermined threshold value: determining in a calibration stage a proportional constant between a load sensitivity and a decreased pressure sensitivity during vehicle turning and storing the proportional constant in a storage means; estimating in an initialization stage a load sensitivity during vehicle turning of an actual running and multiplying the estimated load sensitivity with the proportional constant, stored in advance in the storage means, to thereby automatically set the predetermined threshold value, wherein the load sensitivity is calculated based on a relational expression between a ratio of the dynamic loaded radius change due to the load shift during turning to the dynamic loaded radius, the ratio being calculated based on a yaw rate and a lateral acceleration obtained from a yaw rate sensor and a lateral acceleration sensor mounted in the vehicle and wheel speeds of left and right wheels of the vehicle, and a lateral acceleration. determining in a calibration stage a proportional constant between a load sensitivity and a decreased pressure sensitivity during vehicle turning and storing the proportional constant in a storage means; estimating in an initialization stage a load sensitivity during vehicle turning of an actual running and multiplying the estimated load sensitivity with the proportional constant, stored in advance in the storage means, to thereby automatically set the predetermined threshold value, wherein the load sensitivity is calculated based on a relational expression between a ratio of the dynamic loaded radius change due to the load shift during turning to the dynamic loaded radius, the ratio being calculated based on a yaw rate and a lateral acceleration obtained from a yaw rate sensor and a lateral acceleration sensor mounted in the vehicle and wheel speeds of left and right wheels of the vehicle, and a lateral acceleration.