An impedance detector for detecting an increase of impedance in at least one cable (13A; 13B; 13C, 13D) comprised in a shape measuring instrument (100; 200; 300; 400) comprising: a calculator configured to output test data (TH; TL) through said at least one cable (13A; 13B; 13C, 13D) to a movable portion of the shape measuring instrument (100; 200; 300; 400) having the movable portion, the movable portion configured to output a signal indicating results of an operation, wherein: the cable (13A; 13B; 13C, 13D) is configured to bend in accordance with displacement of the movable portion, the calculator is configured to detect an increase of impedance in the cable (13A; 13B; 13C, 13D) according to whether a reception confirmation signal (Sd) output when the movable portion receives the test data (TH; TL) is confirmed to be received through the cable (13A; 13B; 13C, 13D), wherein the calculator is configured to: output the test data (TH; TL) to the movable portion N times, where N is an integer equal to or greater than 1; output a pass determination when the reception confirmation signal (Sd) is received at least M times thereby determining that there is no increase of impedance, where M is an integer equal to or greater than 1 and equal to or less than N, and output a fail determination when the reception confirmation signal (Sd) is received less than M times thereby determining that there is an increase of impedance, where M is an integer equal to or greater than 1 and equal to or less than N; and wherein a communication speed of the test data (TH; TL) transmitted from the calculator, through the cable (13A; 13B; 13C, 13D), and to the movable portion is set to be higher than a communication speed of the signal indicating the results of the operation and transmitted from the movable portion, through the cable (13A; 13B; 13C, 13D), and to a controller of the shape measuring instrument (100; 200; 300; 400) having the movable portion. a calculator configured to output test data (TH; TL) through said at least one cable (13A; 13B; 13C, 13D) to a movable portion of the shape measuring instrument (100; 200; 300; 400) having the movable portion, the movable portion configured to output a signal indicating results of an operation, wherein: the cable (13A; 13B; 13C, 13D) is configured to bend in accordance with displacement of the movable portion, the calculator is configured to detect an increase of impedance in the cable (13A; 13B; 13C, 13D) according to whether a reception confirmation signal (Sd) output when the movable portion receives the test data (TH; TL) is confirmed to be received through the cable (13A; 13B; 13C, 13D), the cable (13A; 13B; 13C, 13D) is configured to bend in accordance with displacement of the movable portion, the calculator is configured to detect an increase of impedance in the cable (13A; 13B; 13C, 13D) according to whether a reception confirmation signal (Sd) output when the movable portion receives the test data (TH; TL) is confirmed to be received through the cable (13A; 13B; 13C, 13D), output the test data (TH; TL) to the movable portion N times, where N is an integer equal to or greater than 1; output a pass determination when the reception confirmation signal (Sd) is received at least M times thereby determining that there is no increase of impedance, where M is an integer equal to or greater than 1 and equal to or less than N, and output a fail determination when the reception confirmation signal (Sd) is received less than M times thereby determining that there is an increase of impedance, where M is an integer equal to or greater than 1 and equal to or less than N; and wherein a communication speed of the test data (TH; TL) transmitted from the calculator, through the cable (13A; 13B; 13C, 13D), and to the movable portion is set to be higher than a communication speed of the signal indicating the results of the operation and transmitted from the movable portion, through the cable (13A; 13B; 13C, 13D), and to a controller of the shape measuring instrument (100; 200; 300; 400) having the movable portion.