PURPOSE:To increase the density of a key matrix compared with the number of key switches and to decrease the number of wirings, by applying a key detecting system using a 3-dimensional key matrix. CONSTITUTION:The 1st decoder 7 first scans successively common axes y1c, y2c and y3c of a (y) axis group at L levels. When a key switch s1 connected to the intersecting points of x1, y1 and z1 axes is pushed, a common axis x1c of an x1 axis group is set at an L level via the switch s1 when the axis y1c is set at an L level through the scan of the decoder 7. While a control circuit 6 detects the push of either one of key switches s1, s2 and s3 connected to the intersecting point between the x1 and y1 axis groups. Then the decoder 7 sets all common axes of the (y) axis group at H levels. The 2nd decoder 8 scans successively common axes z1c, z2c and z3c of a (z) axis group at L levels. The the axis x1c of the x1 axis group is set again at an L level through the switch s1 when the axis z1c is set at an L level.
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