A zero-crossing detector circuit comprising: a first capacitor including a first electrode (C1p1) configured to connect to one end of an AC power supply and a second electrode (C1p2); a second capacitor including a first electrode (C2p1) configured to connect to the other end of the AC power supply and a second electrode (C2p2); a current path (41; 41C); which is connected in series between the second electrode (C1p2) of the first capacitor and the second electrode (C2p2) of the second capacitor, and which is connected to a reference potential (Vgd), and wherein the current path (41; 41C) causes an AC current output from the AC power supply to pass through the current path (41; 41C) when the AC current returns to the AC power supply, and generates a second-electrode-side voltage (Vds) when the AC current passes through the current path (41; 41C); a signal converting circuit, which is connected to the current path (41; 41C) to receive the second-electrode-side voltage (Vds) and then converts the second-electrode-side voltage (Vds) into a pulse signal (Pzc); and characterized by a detecting unit, which detects a pulse period (Tp) of the pulse signal (Pzc), and which detects zero-crossing points of an AC voltage of the AC power supply by using the pulse period (Tp), the pulse signal (Pzc) and a known phase shift between the pulse signal and the AC voltage of the AC power supply, wherein the current path (41; 41C) includes a bridge circuit that is configured by a pair of series circuits, each of which includes two circuit elements (D4, D5, D1, D3; R6, R7, D1, D3; D4, D5, R6, R7; D3, D4, R6, R7; C3, C4, D1, D4), wherein a contact node between the circuit elements (D4, D5; R6, R7; R6, D4; C3, D4) of one of the pair of series circuits of the bridge circuit is connected to the reference potential (Vgd), and wherein a contact node between the circuit elements (D1, D3; R6, R7; D3, R7; C4, D1) of the other of the pair of series circuits is connected to the signal converting circuit, wherein, in a case where the pulse period (Tp) of the pulse signal (Pzc) is greater than a predetermined period, the detecting unit detects the zero-crossing points on the basis of rising timings of the pulse signal (Tzc) and falling timings of the pulse signal (Tzc), and wherein, in a case where the pulse period (Tp) of the pulse signal (Tzp) is equal to or less than the predetermined period, the detecting unit detects the zero-crossing points on the basis of either the rising timings of the pulse signal (Tzp) or the falling timings of the pulse signal (Tzp). a first capacitor including a first electrode (C1p1) configured to connect to one end of an AC power supply and a second electrode (C1p2); a second capacitor including a first electrode (C2p1) configured to connect to the other end of the AC power supply and a second electrode (C2p2); a current path (41; 41C); which is connected in series between the second electrode (C1p2) of the first capacitor and the second electrode (C2p2) of the second capacitor, and which is connected to a reference potential (Vgd), and wherein the current path (41; 41C) causes an AC current output from the AC power supply to pass through the current path (41; 41C) when the AC current returns to the AC power supply, and generates a second-electrode-side voltage (Vds) when the AC current passes through the current path (41; 41C); a signal converting circuit, which is connected to the current path (41; 41C) to receive the second-electrode-side voltage (Vds) and then converts the second-electrode-side voltage (Vds) into a pulse signal (Pzc); and characterized by a detecting unit, which detects a pulse period (Tp) of the pulse signal (Pzc), and which detects zero-crossing points of an AC voltage of the AC power supply by using the pulse period (Tp), the pulse signal (Pzc) and a known phase shift between the pulse signal and the AC voltage of the AC power supply, wherein the current path (41; 41C) includes a bridge circuit that is configured by a pair of series circuits, each of which includes two circuit elements (D4, D5, D1, D3; R6, R7, D1, D3; D4, D5, R6, R7; D3, D4, R6, R7; C3, C4, D1, D4), wherein a contact node between the circuit elements (D4, D5; R6, R7; R6, D4; C3, D4) of one of the pair of series circuits of the bridge circuit is connected to the reference potential (Vgd), and wherein a contact node between the circuit elements (D1, D3; R6, R7; D3, R7; C4, D1) of the other of the pair of series circuits is connected to the signal converting circuit, wherein, in a case where the pulse period (Tp) of the pulse signal (Pzc) is greater than a predetermined period, the detecting unit detects the zero-crossing points on the basis of rising timings of the pulse signal (Tzc) and falling timings of the pulse signal (Tzc), and wherein, in a case where the pulse period (Tp) of the pulse signal (Tzp) is equal to or less than the predetermined period, the detecting unit detects the zero-crossing points on the basis of either the rising timings of the pulse signal (Tzp) or the falling timings of the pulse signal (Tzp).