図：特定口座の税金-確定拠出年金の税金

左 5%、右 8%

確定拠出年金は運用益も非課税なので特定口座に追いつくのはかなり難しい

年率10%成長だとしても100万円以上得

code:py

import matplotlib.pyplot as plt

import numpy as np

annual_income = 5000000

annual_social_fee = 750000 # 社会保険料

annual_basic_deduction = 480000 # 基礎控除

years_of_service = 30 # 勤続年数

years_of_investment = 30 # 今から定年までの期間

inital_CDC_value = 0 # 現時点での確定拠出年金の運用額

initial_iDeCo_value = 0 #　現時点でのiDeCoの運用額

annual_growth_rate = 5 # %

initial_DC_value = inital_CDC_value + initial_iDeCo_value

annual_taxable_income = annual_income - annual_social_fee - annual_basic_deduction

# Tax calculation functions

def calculate_progressive_tax(income):

tax = 0

lower_limit = 0

tax_brackets = [

(1950000, 0.15),

(3300000, 0.20),

(6950000, 0.30),

(9000000, 0.33),

(18000000, 0.43),

(40000000, 0.5),

(99999999999, 0.55)

]

for upper_limit, rate in tax_brackets:

if income > upper_limit:

tax += (upper_limit - lower_limit) * rate

else:

tax += (income - lower_limit) * rate

break

lower_limit = upper_limit

return tax

def get_japan_tax_rate(income):

if income <= 1949000:

return 0.15

elif 1950000 <= income <= 3299000:

return 0.2

elif 3300000 <= income <= 6949000:

return 0.3

elif 6950000 <= income <= 8999000:

return 0.33

elif 9000000 <= income <= 17999000:

return 0.43

elif 18000000 <= income <= 39999000:

return 0.5

else: # income >= 40000000

return 0.55

# 退職所得控除

def calculate_retirement_deduction(years):

if years <= 20:

return 400000 * years

else:

return 8000000 + 700000 * (years - 20)

# 退職金にかかる税金

def calculate_retirement_tax(retirement_pay, years):

return calculate_progressive_tax((retirement_pay - calculate_retirement_deduction(years)) / 2)

# Function to calculate the total value of investments

def calculate_total_value(monthly_fee, annual_growth_rate, years_of_service):

asset_from_now_growth = (monthly_fee * ((1 + (annual_growth_rate / 12 / 100)) ** (years_of_investment * 12) - 1)) / (annual_growth_rate / 12 / 100);

# 現在の運用資産の成長

return asset_from_now_growth

# Function to calculate tax benefit from iDeCo

def calculate_ideco_benefit(monthly_ideco_fee, annual_growth_rate, years_of_service):

annual_ideco_fee = monthly_ideco_fee * 12

post_investment_value = calculate_total_value(monthly_ideco_fee, annual_growth_rate, years_of_service)

ideco_total_tax = calculate_retirement_tax(post_investment_value, years_of_service)

# 給与受け取り時に取られる所得税

salaryTax = (annual_ideco_fee * years_of_service) * get_japan_tax_rate(annual_taxable_income)

# 特定口座なので20%税率とられる

regular_total_tax = (post_investment_value - (annual_ideco_fee * years_of_service)) * 0.2 + salaryTax

return regular_total_tax - ideco_total_tax

# Generate graph

monthly_ideco_fees = np.linspace(5000, 55000, 100) # Range of monthly iDeCo fees to explore

plt.figure(figsize=(10, 6))

plt.plot(monthly_ideco_fees, benefits)

plt.xlabel('Monthly iDeCo Fee (Yen)')

plt.ylabel('Tax Benefit Standard - DC (Yen)')

plt.title('Tax Benefit of iDeCo Depending on Monthly Fee')

plt.grid(True)

plt.show()