A catalyst degradation determining device for a straddled vehicle, used in a straddled vehicle having a transmission configured to transmit motive power of an engine, a catalyst configured to clean an exhaust gas from the engine, an upstream oxygen sensor provided at a portion farther upstream than the catalyst, the upstream oxygen sensor is configured to detect an oxygen concentration of an exhaust gas which has been exhausted from the engine and has yet been cleaned by the catalyst and is configured to output an electrical signal indicating the detected oxygen concentration and a downstream oxygen sensor provided at a portion farther downstream than the catalyst, the downstream oxygen sensor is configured to detect an oxygen concentration of an exhaust gas, which has been exhausted from the engine and has been cleaned by the catalyst and is configured to output an electrical signal indicating the detected oxygen concentration, characterized in that catalyst degradation determining device comprising: a running state acquirer configured to acquire each of a gear ratio of the transmission, an intake state of the engine and a rotation speed of the engine as a running state; a state determiner configured to store conditions for a plurality of running states in which execution of determining work in regard to a degradation state of a catalyst is allowed as allowable conditions based on a relationship among a gear ratio of the transmission, an intake state of the engine and a rotation speed of the engine, and to determine whether a plurality of running states acquired by the running state acquirer satisfy the allowable conditions, wherein the determining work includes forcibly increasing a change period and a change range of an air-fuel ratio of an air-fuel mixture caused by a feedback control as compared to a case where the determining work is not executed; a determination executor configured to execute determining work in regard to the degradation state of a catalyst in a case where a plurality of running states acquired by the running state acquirer satisfy the allowable conditions, and configured to not execute determining work in regard to the degradation state of a catalyst in a case where a plurality of running states acquired by the running state acquirer do not satisfy the allowable conditions; and a degradation determiner configured to determine a degradation state of a catalyst based on changes of oxygen concentrations that are output from the upstream oxygen sensor and the downstream oxygen sensor acquired by the running state acquirer. a running state acquirer configured to acquire each of a gear ratio of the transmission, an intake state of the engine and a rotation speed of the engine as a running state; a state determiner configured to store conditions for a plurality of running states in which execution of determining work in regard to a degradation state of a catalyst is allowed as allowable conditions based on a relationship among a gear ratio of the transmission, an intake state of the engine and a rotation speed of the engine, and to determine whether a plurality of running states acquired by the running state acquirer satisfy the allowable conditions, wherein the determining work includes forcibly increasing a change period and a change range of an air-fuel ratio of an air-fuel mixture caused by a feedback control as compared to a case where the determining work is not executed; a determination executor configured to execute determining work in regard to the degradation state of a catalyst in a case where a plurality of running states acquired by the running state acquirer satisfy the allowable conditions, and configured to not execute determining work in regard to the degradation state of a catalyst in a case where a plurality of running states acquired by the running state acquirer do not satisfy the allowable conditions; and a degradation determiner configured to determine a degradation state of a catalyst based on changes of oxygen concentrations that are output from the upstream oxygen sensor and the downstream oxygen sensor acquired by the running state acquirer.