To improve an eye opening by thinning a base line.SOLUTION: A PAM4 decoder 1 comprises: three 0/1 discrimination devices 2a, 2b, and 2c that perform a 0/1 discrimination of a high level signal, a middle level signal, and a low level signal, each which has a different amplitude level to a base line of a PAM4 signal; and a decode circuit 3 which is structured by a wired OR formed by an OR logic wiring an output of the three 0/1 discrimination devices 2a, 2b, and 2c view a transmission line, and which decodes a discrimination signal from the three 0/1 discrimination devices 2a, 2b, and 2c to an uppermost significant bit string signal MSB and a lowermost bit string signal LSB. When a load resistance on an input side in the wired OR is R1, the load resistance on an output side is R4, a characteristic impedance of the transmission line on the input side is Z2, and a characteristic impedance of the transmission line on the output side is Z3, a constant number of the characteristic impedance of the transmission line on the input side and the output side is set so as to satisfy the followings: R1≠R4, |Z2-R1||Z3-R4|, and Z2Z3.SELECTED DRAWING: Figure 1
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