A target object measuring device, comprising: a transducer for transmitting a transmission wave underwater and receiving a reflection wave; a target object echo signal extracting module for extracting an echo signal indicative of a target object from data of the reflection wave; a peak detecting module for detecting each of a plurality of peaks (P1, P2) included in the echo signal; a peak incoming direction calculating module for calculating an incoming direction of each of the plurality of peaks (P1, P2) based on the reflection wave; characterized by: a time difference calculating module for calculating a time difference between a timing of appearance of a first peak and a second peak different from the first peak, among the plurality of peaks (P1, P2), a difference in the incoming direction between the first peak and the second peak being smaller than a given threshold; and a target object measuring module for calculating data related to a size and/or a weight of the target object based on the time difference, wherein the time difference calculating module generates a histogram based on the time difference calculated for a plurality of the echo signals, and wherein the target object measuring module calculates the data related to the size and/or the weight of the target object based on the histogram. a transducer for transmitting a transmission wave underwater and receiving a reflection wave; a target object echo signal extracting module for extracting an echo signal indicative of a target object from data of the reflection wave; a peak detecting module for detecting each of a plurality of peaks (P1, P2) included in the echo signal; a peak incoming direction calculating module for calculating an incoming direction of each of the plurality of peaks (P1, P2) based on the reflection wave; characterized by: a time difference calculating module for calculating a time difference between a timing of appearance of a first peak and a second peak different from the first peak, among the plurality of peaks (P1, P2), a difference in the incoming direction between the first peak and the second peak being smaller than a given threshold; and a target object measuring module for calculating data related to a size and/or a weight of the target object based on the time difference, wherein the time difference calculating module generates a histogram based on the time difference calculated for a plurality of the echo signals, and wherein the target object measuring module calculates the data related to the size and/or the weight of the target object based on the histogram. a time difference calculating module for calculating a time difference between a timing of appearance of a first peak and a second peak different from the first peak, among the plurality of peaks (P1, P2), a difference in the incoming direction between the first peak and the second peak being smaller than a given threshold; and a target object measuring module for calculating data related to a size and/or a weight of the target object based on the time difference, wherein the time difference calculating module generates a histogram based on the time difference calculated for a plurality of the echo signals, and wherein the target object measuring module calculates the data related to the size and/or the weight of the target object based on the histogram.