A biological signal detector comprising: a mechanical amplification device provided with a three-dimensional knitted material for generating string vibration by vibration propagation involved in a human biological signal and a plate-shaped foam body (21,22) stacked on both the front side and the back side of the three-dimensional knitted material for generating membrane vibration by the vibration propagation involved in the human biological signal, and for converting the vibration involved in the human biological signal to amplified solid vibration by a superposing action of the string vibration and membrane vibration; and a vibration sensor attached to the mechanical amplification device for detecting the amplified solid vibration; wherein the three-dimensional knitted material includes a pair of ground knitted fabrics arranged separately from each other and a large number of connecting yarns which reciprocate between the pair of ground knitted fabrics and connect the both; the three-dimensional knitted material has a spring constant close to a spring constant obtained from the load-deflection characteristic of a human muscle in a load-deflection characteristic within a range of a load up to 100 N when being pressurized by a pressure plate having a diameter of 30 mm or a diameter of 98 mm; each plate-shaped foam body (21,22) is a bead foam body; the mechanical amplification device has a film laminated between the three-dimensional knitted material and at least one of the plate-shaped foam bodies (21,22) and the membrane vibration of the film is further superposed to the string vibration and the membrane vibration; the mechanical amplification device is further provided with a three-dimensional knitted material supporting member in which a through hole for arrangement to arrange the three-dimensional knitted material is formed; in a state where the three-dimensional knitted material is arranged in the through hole for arrangement, the film is laminated on at least either one of the front side and the back side of the three-dimensional knitted material, and a peripheral edge portion of the film is fixed to the three-dimensional knitted material supporting member; each plate-shaped foam body (21,22) that has a film laminated between that plate-shaped foam body and the three-dimensional knitted material is stacked through the film; the three-dimensional knitted material supporting member is a bead foam body formed having a plate shape; and the three-dimensional knitted material has the spring constant within a range of 0.1 to 5 N/mm when being pressurized by the pressure plate having the diameter of 30 mm or the spring constant within a range of 1 to 10 N/mm when being pressurized by the pressure plate having the diameter of 98 mm. a mechanical amplification device provided with a three-dimensional knitted material for generating string vibration by vibration propagation involved in a human biological signal and a plate-shaped foam body (21,22) stacked on both the front side and the back side of the three-dimensional knitted material for generating membrane vibration by the vibration propagation involved in the human biological signal, and for converting the vibration involved in the human biological signal to amplified solid vibration by a superposing action of the string vibration and membrane vibration; and a vibration sensor attached to the mechanical amplification device for detecting the amplified solid vibration; wherein the three-dimensional knitted material includes a pair of ground knitted fabrics arranged separately from each other and a large number of connecting yarns which reciprocate between the pair of ground knitted fabrics and connect the both; the three-dimensional knitted material has a spring constant close to a spring constant obtained from the load-deflection characteristic of a human muscle in a load-deflection characteristic within a range of a load up to 100 N when being pressurized by a pressure plate having a diameter of 30 mm or a diameter of 98 mm; each plate-shaped foam body (21,22) is a bead foam body; the mechanical amplification device has a film laminated between the three-dimensional knitted material and at least one of the plate-shaped foam bodies (21,22) and the membrane vibration of the film is further superposed to the string vibration and the membrane vibration; the mechanical amplification device is further provided with a three-dimensional knitted material supporting member in which a through hole for arrangement to arrange the three-dimensional knitted material is formed; in a state where the three-dimensional knitted material is arranged in the through hole for arrangement, the film is laminated on at least either one of the front side and the back side of the three-dimensional knitted material, and a peripheral edge portion of the film is fixed to the three-dimensional knitted material supporting member; each plate-shaped foam body (21,22) that has a film laminated between that plate-shaped foam body and the three-dimensional knitted material is stacked through the film; the three-dimensional knitted material supporting member is a bead foam body formed having a plate shape; and the three-dimensional knitted material has the spring constant within a range of 0.1 to 5 N/mm when being pressurized by the pressure plate having the diameter of 30 mm or the spring constant within a range of 1 to 10 N/mm when being pressurized by the pressure plate having the diameter of 98 mm.