A power conversion device comprising: a cell unit configured to supply AC power having a variable frequency to an electric motor; a first sensor (VM; AM; TM; 7) provided for the cell unit, the first sensor (VM; AM; TM; 7) being configured to measure a value relating to a first monitoring item of the cell unit; a second sensor (VM; AM; TM; 7) provided for the cell unit, the second sensor (VM; AM; TM; 7) being configured to measure a value relating to a second monitoring item of the cell unit; a storage; a first controller configured to monitor an output result of the first sensor (VM; AM; TM; 7) and an output result of the second sensor (VM; AM; TM; 7) in both a first state in which a sign of a malfunction has not occurred in the power conversion device and a second state in which the sign of the malfunction has occurred in the power conversion device, and to control or protect the cell unit on the basis of at least one of the output result of the first sensor (VM; AM; TM; 7) and the output result of the second sensor (VM; AM; TM; 7); a second controller configured to, in a case in which the cell unit is in the first state, store the output result of the first sensor (VM; AM; TM; 7) measured at each first sampling cycle in the storage over at least a first period without storing the output result of the second sensor (VM; AM; TM; 7) in the storage, and configured to, in a case in which the cell unit transitions to the second state in which a change in at least one of the output result of the first sensor (VM; AM; TM; 7) and the output result of the second sensor (VM; AM; TM; 7) satisfies a first condition, execute a third operation of storing the output result of the second sensor (VM; AM; TM; 7) in the storage instead of the output result of the first sensor (VM; AM; TM; 7), wherein the first monitoring item is different from the second monitoring item. a cell unit configured to supply AC power having a variable frequency to an electric motor; a first sensor (VM; AM; TM; 7) provided for the cell unit, the first sensor (VM; AM; TM; 7) being configured to measure a value relating to a first monitoring item of the cell unit; a second sensor (VM; AM; TM; 7) provided for the cell unit, the second sensor (VM; AM; TM; 7) being configured to measure a value relating to a second monitoring item of the cell unit; a storage; a first controller configured to monitor an output result of the first sensor (VM; AM; TM; 7) and an output result of the second sensor (VM; AM; TM; 7) in both a first state in which a sign of a malfunction has not occurred in the power conversion device and a second state in which the sign of the malfunction has occurred in the power conversion device, and to control or protect the cell unit on the basis of at least one of the output result of the first sensor (VM; AM; TM; 7) and the output result of the second sensor (VM; AM; TM; 7); a second controller configured to, in a case in which the cell unit is in the first state, store the output result of the first sensor (VM; AM; TM; 7) measured at each first sampling cycle in the storage over at least a first period without storing the output result of the second sensor (VM; AM; TM; 7) in the storage, and configured to, in a case in which the cell unit transitions to the second state in which a change in at least one of the output result of the first sensor (VM; AM; TM; 7) and the output result of the second sensor (VM; AM; TM; 7) satisfies a first condition, execute a third operation of storing the output result of the second sensor (VM; AM; TM; 7) in the storage instead of the output result of the first sensor (VM; AM; TM; 7), wherein the first monitoring item is different from the second monitoring item.