Target strength (TS), mantle length (ML), swimming speed, and tilt angle of Boreopacific gonate squid Gonatopsis borealis were measured using an acoustical‐optical composite system (J‐QUEST) in the North Pacific Ocean. The J‐QUEST consists of a quantitative echosounder operating at 70 kHz and a high‐sensitivity stereo video camera system installed in a housing resistant to pressure at the depth of 300 m. In the daytime, Boreopacific gonate squids were seen at the J‐QUEST depth of 230 m by the stereo video camera. Swift vertical movements were also observed on the echogram at the same time. Swimming speeds and tilt angles were estimated by tracking echoes from the same squid. Estimated average swimming speed was over 1 m/s and tilt angles were almost vertical. Average TS calculated from squidlike echoes was −54.1 dB and average ML calculated from the stereo video images was 23.9 cm with standard deviation of 5.9 cm. Standardized TS by the squared ML was −81.9 dB. In the nighttime, a Boreopacific gonate squid (estimated ML 37 cm) at the J‐QUEST depth of 25 m was observed preying on one Japanese anchovy, Engraulis japonicus, in an aggregation with estimated average length of 12.3 cm.