Although measurement of abdominal strength is important for evaluating trunk myofunction, adequate devices for these measurements have not been developed. As a consequence, the relationships between abdominal strength and abdominal muscle thickness have also not been determined. This study aimed to develop a new device for measuring abdominal strength and to examine the relationship between abdominal muscle strength and thickness. The subjects in the study included 50 young males (age, 19.1 ± 0.32 years; height, 171.0 ± 6.55 cm; weight, 69.5 ± 12.15 kg). The measurement of abdominal flexion strength had high reliability with an intra-class correlation coefficient (ICC) >0.9. The thickness of the rectus abdominis muscle was measured under two conditions using an ultrasound imaging method; relaxation and strength exertion. Abdominal muscle thickness was significantly thicker during exertion than during relaxation. Abdominal flexion strength showed a significant correlation with muscle thickness during both relaxation (r = 0.69) and strength exertion (r = 0.71). No significant difference was found between these correlation coefficients. In conclusion, the values of abdominal strength determined by our newly-developed measuring device had high reliability, with abdominal flexion strength showing a relatively close relationship with the thickness of the rectus abdominis during strength exertion and relaxation.