The purpose of the present study was to investigate the factors which were expected to have some relations to the differences between back strength measurements obtained by using spring and strain gauge installed dynamometers. Several mechanical properties of both types of dynamometer, differences between measurements of the dynamometers, and optimum incline angle of upper body to gain the maximum back strength were examined respectively. The results obtained here were as follows: 1) Though there was a slight delay on the peak of the measurement of spring dynamometer, there were no factors which influenced to the measurements under conditions using carefully calibrated dynamometers. 2) No difference between two measurements was obtained for low strength subject group (<110kg). Meanwhile, for high strength group (≧110kg), measurements of spring dynamometer took larger values than those of strain gauge dynamometer. And larger strength subject tended to take larger difference of the measurements. 3) Maximum back strength measurement was obtained at 30 degrees of upper body incline angle when the spring dynamometer was used. On the other hand, 20 degrees for the strain gauge dynamometer was obtained. 4) From the consideration of the above results, it was concluded that the upper body should be set to different incline angles to measure the maximum value when the different types of dynamometer are used.