using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class FireMissile : MonoBehaviour
{
    public GameObject missilePrefab;

    // ★変更（privateに変更）
    // 初期の速度は遅くすること
    private float speed = 300;

    public AudioClip sound;
    private int count;

    void FixedUpdate()
    {
        if (Input.GetKey(KeyCode.Space))
        {
            count += 1;

            if (count % 5 == 0)
            {
                GameObject missile = Instantiate(missilePrefab, transform.position, Quaternion.identity);
                Rigidbody missileRb = missile.GetComponent<Rigidbody>();
                missileRb.AddForce(transform.forward * speed);
                AudioSource.PlayClipAtPoint(sound, Camera.main.transform.position);
                Destroy(missile, 2.0f);
            }
        }
    }

    // ★追加
    public void AddShotSpeed(int amount)
    {
        speed += amount;

        // 上限の設定（上限をどこにするかは自由）
        if(speed > 1000)
        {
            speed = 1000;
        }
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ShotSpeedUp : ItemBase
{
    private void OnTriggerEnter(Collider other)
    {
        if(other.CompareTag("Player"))
        {
            ItemGet();

            // Playerの子オブジェクトのコンポーネントを取得する（ポイント）
            // １個のアイテムゲットでどれだけ速度を上げるかは自由
            other.GetComponentInChildren<FireMissile>().AddShotSpeed(300);
        }
    }
}