A system, comprising at least two exhaust sensors (43, 45, 47, 49, 51) for mounting around a catalyst or catalytic converter disposed in an exhaust passage of an internal combustion engine, an engine controlling device for controlling said internal combustion engine, a digital communication line and a sensor control device for controlling the at least two exhaust sensors (43, 45, 47, 49, 51), the sensor control device comprising: a plurality of drive control circuits connected in a one-to-one relation with said exhaust sensors, each of said plurality of drive control circuits being for driving/controlling respective exhaust sensors (43, 45, 47, 49, 51) and being for outputting sensor signals, each of said sensor signals corresponding to respective electric signals outputted by each of said exhaust sensors; a microcomputer connected with said plurality of drive control circuits, said microcomputer being for acquiring said sensor signals; and a communication output section for outputting a plurality of signals comprising at least one of said sensor signals acquired by said microcomputer, to the engine controlling device through the digital communication line, wherein said sensor control device further comprises a casing that houses said plurality of drive control circuits, said microcomputer and said communication output section, wherein said exhaust sensors comprises at least one weak signal output sensor having a weak signal which has one of a maximum current value of 1 [mA] or less and a maximum voltage of 100 [mV] or less, as a first electric signal which is one of said electric signals, wherein the sensor control device further comprises a plurality of sensor connecting connectors connected in a one-to-one relationship with respective connectors of said exhaust sensors so as to form, together with signal cables (H1, H2), a signal passage connecting said plurality of drive control circuits with said exhaust sensors (43, 45, 47, 49, 51), wherein each of said plurality of sensor connecting connectors removably connects a respective one of the connectors of said exhaust sensors, wherein the signal cables (H1, H2) connecting the exhaust sensors with the respective drive control circuits are shorter than the digital communication line, wherein said microcomputer includes a catalyst state detecting section for detecting a state of said catalyst or catalytic converter on the basis of at least one of said sensor signals, and for outputting a catalyst state signal indicating the catalyst state detected, to said communication output section, and wherein said communication output section is adapted for outputting said catalyst state signal to said engine controlling device through said digital communication line. a plurality of drive control circuits connected in a one-to-one relation with said exhaust sensors, each of said plurality of drive control circuits being for driving/controlling respective exhaust sensors (43, 45, 47, 49, 51) and being for outputting sensor signals, each of said sensor signals corresponding to respective electric signals outputted by each of said exhaust sensors; a microcomputer connected with said plurality of drive control circuits, said microcomputer being for acquiring said sensor signals; and a communication output section for outputting a plurality of signals comprising at least one of said sensor signals acquired by said microcomputer, to the engine controlling device through the digital communication line, wherein said sensor control device further comprises a casing that houses said plurality of drive control circuits, said microcomputer and said communication output section, wherein said exhaust sensors comprises at least one weak signal output sensor having a weak signal which has one of a maximum current value of 1 [mA] or less and a maximum voltage of 100 [mV] or less, as a first electric signal which is one of said electric signals, wherein the sensor control device further comprises a plurality of sensor connecting connectors connected in a one-to-one relationship with respective connectors of said exhaust sensors so as to form, together with signal cables (H1, H2), a signal passage connecting said plurality of drive control circuits with said exhaust sensors (43, 45, 47, 49, 51), wherein each of said plurality of sensor connecting connectors removably connects a respective one of the connectors of said exhaust sensors, wherein the signal cables (H1, H2) connecting the exhaust sensors with the respective drive control circuits are shorter than the digital communication line, wherein said microcomputer includes a catalyst state detecting section for detecting a state of said catalyst or catalytic converter on the basis of at least one of said sensor signals, and for outputting a catalyst state signal indicating the catalyst state detected, to said communication output section, and wherein said communication output section is adapted for outputting said catalyst state signal to said engine controlling device through said digital communication line.