A hydraulic energy recovery apparatus for a working machine comprising: a primer mover; a main pump that is driven by the prime mover mountable on a working machine and delivers a hydraulic fluid as pressurized oil; a hydraulic actuator that is driven by the pressurized oil delivered from the main pump; an accumulator that recovers a part or all of returned oil from the hydraulic actuator; a pressure detector that detects a pressure of the accumulator; a reset device that is reset at the time of the replacement of the accumulator; an operation lever device configured to operate the hydraulic actuator; and a controller to which signals from the operation lever device, the pressure detector and the reset device are input, wherein the controller includes: an elapse time measuring section that measures a time elapsed since an initial use of the accumulator based upon the signal from the reset device; and a number-of-operations measuring section that measures a number of operations of the accumulator based upon the signal from the pressure detector; characterized in that the hydraulic energy recovery apparatus further comprises a temperature detector that detects a temperature of the hydraulic fluid, wherein the controller further includes a gas permeation amount estimating section that estimates a gas permeation amount of the accumulator based upon outputs from the elapse time measuring section, the pressure detector and the temperature detector; a sealed gas pressure estimating section that estimates a sealed gas pressure of the accumulator from a rising state of an accumulator pressure in a case of starting accumulation from a state where the accumulator pressure is equal to a tank pressure, based upon the signal from the pressure detector; and an accumulator degradation determining section that determines a degradation condition of the accumulator based upon at least one output of outputs from the elapse time measuring section, the number-of-operations measuring section, the gas permeation amount estimating section, and the sealed gas pressure estimating section, and outputs the determination result. a primer mover; a main pump that is driven by the prime mover mountable on a working machine and delivers a hydraulic fluid as pressurized oil; a hydraulic actuator that is driven by the pressurized oil delivered from the main pump; an accumulator that recovers a part or all of returned oil from the hydraulic actuator; a pressure detector that detects a pressure of the accumulator; a reset device that is reset at the time of the replacement of the accumulator; an operation lever device configured to operate the hydraulic actuator; and a controller to which signals from the operation lever device, the pressure detector and the reset device are input, wherein the controller includes: an elapse time measuring section that measures a time elapsed since an initial use of the accumulator based upon the signal from the reset device; and a number-of-operations measuring section that measures a number of operations of the accumulator based upon the signal from the pressure detector; characterized in that the hydraulic energy recovery apparatus further comprises a temperature detector that detects a temperature of the hydraulic fluid, wherein the controller further includes a gas permeation amount estimating section that estimates a gas permeation amount of the accumulator based upon outputs from the elapse time measuring section, the pressure detector and the temperature detector; a sealed gas pressure estimating section that estimates a sealed gas pressure of the accumulator from a rising state of an accumulator pressure in a case of starting accumulation from a state where the accumulator pressure is equal to a tank pressure, based upon the signal from the pressure detector; and an accumulator degradation determining section that determines a degradation condition of the accumulator based upon at least one output of outputs from the elapse time measuring section, the number-of-operations measuring section, the gas permeation amount estimating section, and the sealed gas pressure estimating section, and outputs the determination result. an elapse time measuring section that measures a time elapsed since an initial use of the accumulator based upon the signal from the reset device; and a number-of-operations measuring section that measures a number of operations of the accumulator based upon the signal from the pressure detector; characterized in that the hydraulic energy recovery apparatus further comprises a temperature detector that detects a temperature of the hydraulic fluid, wherein the controller further includes a gas permeation amount estimating section that estimates a gas permeation amount of the accumulator based upon outputs from the elapse time measuring section, the pressure detector and the temperature detector; a sealed gas pressure estimating section that estimates a sealed gas pressure of the accumulator from a rising state of an accumulator pressure in a case of starting accumulation from a state where the accumulator pressure is equal to a tank pressure, based upon the signal from the pressure detector; and an accumulator degradation determining section that determines a degradation condition of the accumulator based upon at least one output of outputs from the elapse time measuring section, the number-of-operations measuring section, the gas permeation amount estimating section, and the sealed gas pressure estimating section, and outputs the determination result.