An authentication apparatus comprising: a biometric data acquirer configured to acquire fingerprint data and an electrocardiogram, ECG, waveform of a user; a humidity level acquirer configured to acquire a humidity level of skin of the user; a similarity extractor configured to determine a first similarity between the fingerprint data and reference fingerprint data of a pre-registered user, and a second similarity between the ECG waveform and a reference ECG waveform of the pre-registered user; calculate a first difference value between the first similarity and a first threshold, and a second difference value between the second similarity and a second threshold; and determine a combined similarity by applying a first weighted value to the first difference value, applying a second weighted value to the second difference value, and adding the weighted first difference value to the weighted second difference value, the first weighting value having a negative relationship with the humidity level, the second weighting value having a positive relationship with the humidity level, and a sum of the first weighting value and the second weighting value being constant irrespective of the humidity level; and an authenticator configured to authenticate whether the user is the pre-registered user based on the combined similarity. a biometric data acquirer configured to acquire fingerprint data and an electrocardiogram, ECG, waveform of a user; a humidity level acquirer configured to acquire a humidity level of skin of the user; a similarity extractor configured to determine a first similarity between the fingerprint data and reference fingerprint data of a pre-registered user, and a second similarity between the ECG waveform and a reference ECG waveform of the pre-registered user; calculate a first difference value between the first similarity and a first threshold, and a second difference value between the second similarity and a second threshold; and determine a combined similarity by applying a first weighted value to the first difference value, applying a second weighted value to the second difference value, and adding the weighted first difference value to the weighted second difference value, the first weighting value having a negative relationship with the humidity level, the second weighting value having a positive relationship with the humidity level, and a sum of the first weighting value and the second weighting value being constant irrespective of the humidity level; and determine a first similarity between the fingerprint data and reference fingerprint data of a pre-registered user, and a second similarity between the ECG waveform and a reference ECG waveform of the pre-registered user; calculate a first difference value between the first similarity and a first threshold, and a second difference value between the second similarity and a second threshold; and determine a combined similarity by applying a first weighted value to the first difference value, applying a second weighted value to the second difference value, and adding the weighted first difference value to the weighted second difference value, the first weighting value having a negative relationship with the humidity level, the second weighting value having a positive relationship with the humidity level, and a sum of the first weighting value and the second weighting value being constant irrespective of the humidity level; and an authenticator configured to authenticate whether the user is the pre-registered user based on the combined similarity. <EOS>