A power conversion device comprising: a power converter including an arm (5, 6) in which a plurality of converter cells are connected in series, each of the plurality of converter cells including at least two switching elements (31p, 31n), a power storage element, and a pair of output terminals (P1, P2); and a control device configured to control the power converter, the converter cell including a switch to have the converter cell bypassed, wherein the control device is configured to, when the control device senses failure of a converter cell, have a failed converter cell bypassed, estimate an output voltage lost by bypassing the failed converter cell, and have normal converter cells supply the estimated output voltage of the failed converter cell, the control device includes a basic controller configured to provide an arm voltage command value, and individual cell controllers configured to individually control a corresponding converter cell within the arm (5, 6) upon receiving the arm voltage command value, each individual cell controller includes a PWM modulator configured to provide a PWM modulated signal upon receiving a cell voltage command for a corresponding converter cell, characterized in that the individual cell controllers are configured such that individual cell controllers corresponding normal converter cells calculate the voltage command value for the corresponding converter cell based on the arm voltage command value and a signal indicating the magnitude of an additional output voltage, the individual cell controllers include a cell output voltage estimator configured to estimate an output voltage of the corresponding converter cell based on the PWM modulated signal, the individual cell controllers include a first signal switch configured to provide a signal indicating the magnitude of the voltage estimated by the cell output voltage estimator when the corresponding converter cell has failed and to provide a signal indicating a zero voltage when the corresponding converter cell is normal, the control device includes a cell additional voltage calculator configured to provide the signal indicating the magnitude of the additional output voltage allocated to the converter cells by dividing a sum of signals provided from the first signal switch of the individual cell controllers by a total number of all normal converter cells within the arm (5, 6). a power converter including an arm (5, 6) in which a plurality of converter cells are connected in series, each of the plurality of converter cells including at least two switching elements (31p, 31n), a power storage element, and a pair of output terminals (P1, P2); and a control device configured to control the power converter, the converter cell including a switch to have the converter cell bypassed, wherein the control device is configured to, when the control device senses failure of a converter cell, have a failed converter cell bypassed, estimate an output voltage lost by bypassing the failed converter cell, and have normal converter cells supply the estimated output voltage of the failed converter cell, the control device includes a basic controller configured to provide an arm voltage command value, and individual cell controllers configured to individually control a corresponding converter cell within the arm (5, 6) upon receiving the arm voltage command value, a basic controller configured to provide an arm voltage command value, and individual cell controllers configured to individually control a corresponding converter cell within the arm (5, 6) upon receiving the arm voltage command value, each individual cell controller includes a PWM modulator configured to provide a PWM modulated signal upon receiving a cell voltage command for a corresponding converter cell, characterized in that the individual cell controllers are configured such that individual cell controllers corresponding normal converter cells calculate the voltage command value for the corresponding converter cell based on the arm voltage command value and a signal indicating the magnitude of an additional output voltage, the individual cell controllers include a cell output voltage estimator configured to estimate an output voltage of the corresponding converter cell based on the PWM modulated signal, the individual cell controllers include a first signal switch configured to provide a signal indicating the magnitude of the voltage estimated by the cell output voltage estimator when the corresponding converter cell has failed and to provide a signal indicating a zero voltage when the corresponding converter cell is normal, the control device includes a cell additional voltage calculator configured to provide the signal indicating the magnitude of the additional output voltage allocated to the converter cells by dividing a sum of signals provided from the first signal switch of the individual cell controllers by a total number of all normal converter cells within the arm (5, 6).