A differential amplification circuit for detecting a surface muscle action potential is also used as a detection circuit for determining adhesion of a biological electrode to the body surface, and the body of the biological electrode is detected even during the detection of the surface muscle action potential. A muscle evaluation system capable of determining adhesion to a surface is provided. A differential amplification circuit that connects a set of biological electrodes to an inverting input and a non-inverting input and outputs a difference between surface muscle action potentials appearing on the set of biological electrodes, A common mode test signal is output to a common electrode that is in close contact with the body surface in the middle of a pair of biological electrodes, and a common mode test signal having a predetermined signal strength is output from the differential amplifier circuit. It is determined that it is not in close contact with the body surface. [Selection] Figure 1【課題】表面筋活動電位を検出する差動増幅回路を、生体電極の体表面への密着を判定する検出回路に兼用し、表面筋活動電位を検出する間であっても、生体電極の体表面への密着を判定できる筋評価システムを提供する。【解決手段】一組の生体電極を反転入力と非反転入力に接続し、一組の生体電極に表れる表面筋活動電位の差分を出力する差動増幅回路のコモンモード除去機能を利用し、一組の生体電極の中間で体表面に密着するコモン電極にコモンモードテスト信号を出力し、差動増幅回路から所定の信号強度のコモンモードテスト信号が出力されることから、いずれかの生体電極が体表面に密着していないと判定する。【選択図】図１
